Abstract:
Methods and systems are disclosed for forming secure wirebonds between electrical contacts in electronic device assemblies. Representative embodiments of the invention are described for forming a wirebond including system components and method steps for generating electromagnetic energy from a heat source and transmitting heat to a ball formed on a bondwire. Subsequently, pressure applied to the ball at the bonding site is used in the formation of a wirebond.

Description:
TECHNICAL FIELD 
     The invention relates to the manufacture of semiconductor devices. More particularly, the invention relates to methods and systems for forming wirebonds useful for assembling semiconductor devices. 
     BACKGROUND OF THE INVENTION 
     In order to facilitate electrical connections among semiconductor device terminals and external leads, wirebonds are formed by coupling a bondwire to a bonding site. Generally a bondwire is positioned at a bonding site, such as a bond pad on a chip or PCB, while various forces are applied to cause the metals of the wire and bond pad to fuse. The bondwire is usually made of gold, although bondwires of other metals such as aluminum or copper are also used in the semiconductor manufacturing industry. Ball bonding is a common wirebond process. For the purpose of providing context and example, gold ball bonding is discussed herein, although the invention may also be practiced with ball bonding using other materials. During gold ball bonding, a generally spherical or other shape gold nodule, frequently called a “ball” is formed by melting the end of a gold bondwire held by a part of a bonding tool known as a capillary. Capillaries known in the art are typically ceramic axially symmetrical tools with vertical feedholes for feeding the bondwire through the center. Capillaries commonly hold and control the bondwire during the bonding process and are used to exert pressure and transmit ultrasonic energy to the bonding location. 
     The ball is brought into contact with the bond pad. A controlled amount of pressure is then applied by the capillary for a selected amount of time, contributing to the formation of a metallurgical weld between the bondwire and the bond pad as well as deforming the ball into its final shape. The application of pressure interacts with the application of ultrasonic energy as further discussed below. The wirebonding tool severs the wire in preparation for the next wirebond by clamping the wire and raising the capillary. Problems are sometimes encountered in the application of pressure to form the bond. If the pressure is insufficient, a poor bond can result. Excessive pressure can result in damage to associated circuitry. 
     Wirebonding requires that the bondwire and bond pad be placed in intimate contact with each other. Better bonds are achieved when contact is enhanced using ultrasonic energy. The ultrasonic energy is applied as the ball at the end of the bondwire is brought into contact with the bond pad by the capillary, exerting pressure to push the ball against the bond pad. The ultrasonic energy applied by the bonding tool abrades the ball of the bondwire against the surface of the bond pad. This abrading action cleans the bond pad of debris and oxides that may be present, exposing a fresh surface of the bond pad useful for forming a secure bond. The continued application of ultrasonic energy also results in mutual deformation of the bondwire and bond pad where they abrade against one another, which can enhance the metallurgical bond. The amount of ultrasonic energy applied must be carefully controlled. Excessive ultrasonic energy can stress the nascent bond, and the application of insufficient ultrasonic energy fails to adequately enhance bonding. The application of insufficient pressure can also inhibit the effective transmission of ultrasonic energy. 
     When the bondwire and bond pad are made from different metals, such as in the case of wirebonding gold wire to an aluminum bond pad, for example, thermal energy is used to enhance bonding. In effect, heat is used to soften the harder metal, in this example aluminum, to match the hardness of the softer metal, in this example gold. In order to reach the required temperature, heat is applied to the bond pad by heating the underlying surface, such as a PCB. Insufficient heat can inhibit bonding, while excessive heat can result in damage to the circuitry. Some configurations, such as overhangs or stacked die assemblies present particular problems in directing an acceptable amount of heat to the bonding site. 
     Due to these and other problems, it is desirable to avoid unnecessary heating of devices during wirebonding, to efficiently direct thermal energy to desired locations, and to avoid excessive ultrasonic vibrations. Improved wirebonding techniques and systems adapted to provide one or more of these or similar benefits would be useful and advantageous in the arts. 
     SUMMARY OF THE INVENTION 
     In carrying out the principles of the present invention, in accordance with preferred embodiments thereof, preferred embodiments of the invention provide methods and systems for forming secure wirebonds between electrical contacts in semiconductor device assemblies. A method of wirebonding according to the invention includes steps for positioning a bondwire and ball adjacent to a bonding site with a bonding tool, heating the bondwire, ball, and bonding tool, and then placing the ball in contact with the bonding site. 
     According to an aspect of the invention, a preferred embodiment of a method of forming a wirebond includes steps for applying electromagnetic energy to heat a ball formed on a bondwire and subsequently applying pressure to the ball at the bonding site. 
     According to other aspects of the invention, in examples of preferred embodiments, reflected heat is directed to the bondwire, ball, and bonding tool to facilitate bonding. 
     According to yet another aspect of the invention, a system for wirebonding for use in assembling semiconductor devices includes a bonding tool for positioning a bondwire and ball adjacent to a bonding site and for placing the bondwire and ball in contact with the bonding site. A heat source is operably coupled to a heat conduit for heating the bondwire, ball, and bonding tool prior to the formation of a bond. 
     According to yet another aspect of the invention, a wirebonding system includes a heat reflector for reflecting heat to the bondwire from a heat source. 
     According to further aspects of the invention, preferred embodiments of wirebonding systems are disclosed in which a concave heat reflector, such as a mirror, is used to direct heat to bond components. 
     The invention has advantages including but not limited to improved wirebonding methods and systems that efficiently provide heat where required and reduce unwanted heat transfer. Further advantages are realized in the potential reduction of ultrasonic energy and pressure. These and other features, advantages, and benefits of the present invention can be understood by one of ordinary skill in the arts upon careful consideration of the detailed description of representative embodiments of the invention in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more clearly understood from consideration of the following detailed description and drawings in which: 
         FIG. 1A  is a partial side view representing an example of preferred embodiments of systems and method steps for wirebonding according to the invention; 
         FIG. 1B  is a partial side view representing an example of preferred embodiments of systems and further method steps for wirebonding according to the invention; 
         FIG. 2A  is a partial side view representing another example of alternative preferred embodiments of systems and method steps for wirebonding according to the invention; 
         FIG. 2B  is a partial side view representing further method steps for wirebonding according to the invention in the example of alternative embodiments. 
     
    
    
     References in the detailed description correspond to like references in the various drawings unless otherwise noted. Descriptive and directional terms used in the written description such as first, second, top, bottom, upper, side, etc., refer to the drawings themselves as laid out on the paper and not to physical limitations of the invention unless specifically noted. The drawings are not to scale, and some features of embodiments shown and discussed are simplified or amplified for illustrating the principles, features, and advantages of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In general, the invention provides improved wirebonding for the manufacture of integrated circuitry. The preferred embodiments of methods and systems of the invention shown and described by way of example may be used to provide secure wirebonds between electrical contacts of a chip and substrate or leadframe and are particularly useful for forming wirebonds on stress-sensitive or heat-sensitive assemblies, or to bonding sites to which heat does not transmit easily. 
     Referring primarily to  FIG. 1A , a representative embodiment of systems and methods  10  of wirebonding according to the invention is depicted. A PCB  12  has a number of bonding sites  14  for receiving wires  16  for the purpose of making external electrical connections. An unattached wire  18  is shown held by a capillary  20 . The capillary  20  is a tool known in the arts and is a portion of a larger tool used for forming wirebonds. Various equipment may be used for manipulating the unattached wire  18  and PCB  12  within the scope of the invention. The bondwire  18  has a ball  22 , which is positioned adjacent to the bonding site  14  with the bonding tool  20  as known in the arts. A selected area including the ball  22  and preferably the tip  24  of the capillary  20  is heated to create a heated area  26  preliminary to bringing the ball  22  into contact with the bonding site  14 . As shown, a heat source  28 , in this example an electromagnetic heat bulb, is used to produce heat  29  for transmission by a heat conduit  30 , for example a fiber optic cable, to the heated area  26 . The heat conduit  30  preferably has a lens  31  for focusing the transmitted heat  29  on the selected area  26 . A heat reflector  32  is positioned to reflect transmitted heat  33  that initially escapes the heated area  26  back into the heated area  26 . As shown in the preferred embodiment  10  of  FIGS. 1A and 1B , the heat reflector  32  may be concave. In the presently preferred embodiment, a concave mirror  32  is used, although a flat mirror, or other concave or flat panel of reflective material may also be used. 
     A contact heat source  35  may also be used to apply heat indirectly to the bonding site  14  as is common in the arts, typically placed in contact with the PCB  12 . As shown in  FIG. 1B , The previously heated ball  22  is pressed into contact with the bonding site  14 , using the bonding tool  20  to apply a predetermined amount of compressing force  36 . A predetermined amount of ultrasonic energy  38  is also applied to the ball  22  and bonding site  14  as is known in the art for forming wirebonds. The ball  22  deforms under the pressure of the compressing force  36  producing the final shape of the completed wirebond  44 . 
     It should be appreciated by those skilled in the arts that using the invention, the compressing force  36  used to press on the ball  22 , and/or the ultrasonic energy  38  applied to the bonding site  14  and ball  22 , may in some instances be reduced from the amount otherwise required. Additionally or alternatively, the duration of the application of the compressing force  36 , or ultrasonic energy  38 , or both, may be reduced below levels previously known in the arts in some applications without detriment to producing a secure wirebond  44 . It should also be understood that heat is applied precisely where needed to promote bonding without making physical contact with the device  46  undergoing assembly. 
     An alternative depiction of methods and systems  10  for wirebonding in the assembly of semiconductor devices is shown in  FIGS. 2A and 2B . The bonding tool  20  is used for positioning a bondwire  18  and ball  22  adjacent to a bonding site  14  and for subsequently placing the ball  22  in contact with the bonding site  14 . The ball  22  portion of the bondwire  18  and the tip  24  of the bonding tool  20  are preheated as described by a heat source  28 , conduit  30 , and reflector  32 . Additional heat may be provided by an indirect heat source  35  in contact with the PCB  12 . As shown, the semiconductor device  46  may have one or more stacked PCBs  12 . Also, one or more of the PCBs  12  may have an overhanging portion  48 . It has been found that overhangs  48  may in some instances be less susceptible to heat transmission from an indirect heat source  35 , and that overhangs may also be more likely to be damaged by relatively large amounts of ultrasonic energy  38  or compression force  36 . The use of the invention as shown and described permits advantageous reductions in the exposure of such overhangs to potential damage. 
     The methods and systems of the invention provide advantages including but not limited to improved wirebonding in IC packages, avoiding transfer of misplaced or excessive heat to nearby structures, and permitting a reduction in ultrasonic energy required for forming a secure wirebond. While the invention has been described with reference to certain illustrative embodiments, those described herein are not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other advantages and embodiments of the invention will be apparent to persons skilled in the arts upon reference to the drawings, description, and claims.