Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates generally to flip chip packaging technologies for integrated circuits more particularly to a methodology and trace design for minimizing electromigration damage to solder bumps in a ball grid array or flip-chip package. 
     Electromigration is the movement of material within a conductor that is caused by the flow of electrical current. Electromigration can cause the complete depletion of material within a conductor leading to the loss of continuity. The effect is more apparent at interconnect junctions, for example, in a solder bump connecting a flip-chip die and substrate, and is dependent on the current density (higher being worse than lower), the material (some materials resisting the effects of electromigration more than others), and the geometry of the structure. 
     Electromigration is a problem commonly seen in high-current-flow bumps of flip-chip assemblies, so named because during formation, the die pads are formed on the top layer of the integrated circuit die, bumps are added, and the die is then “flipped” over and connected directly to the chip substrate via the bumps. More specifically, and with reference to  FIGS. 1 and 2 , circuit components are formed on a semiconductor wafer using standard fabrication techniques, with local interconnect layers (formed of interleaved metal and dielectric layers) situated closer to the functional circuitry and global interconnect layers formed further up the sequence of layers. Die pads  22  are formed in the uppermost metal layer. Bumps are then added, and the wafer is diced into individual integrated circuit die  14  for packaging. An individual die  14  is then “flipped” over and attached directly to a substrate  12  or board through the bumps  16 , as shown in  FIG. 1 . 
     Bumps  16  are formed through one of several different processes, including solder bumping, using processes that are well known in the art.  FIG. 2  illustrates a portion of a flip-chip assembly  10  which utilizes solder bumps  16 . In the solder bumping process, an under bump metallization (UBM)  26  is applied to the chip bond pads, by sputtering, plating, or other means, to replace the insulating passivation layer  24  (typically comprising a polymer such as Benzoclyclobutene or “BCB”) typically applied over the top metal layer, and to define and limit the solder-wetted area. Solder is deposited over the UBM  26  by evaporation, electroplating, screen printing solder paste, or needle-depositing. 
       FIG. 1  illustrates an example of a typical path of current flow  18  in a flip-chip assembly  10  that utilizes a conductive bump  16  for interconnecting the pads (not visible) of an integrated circuit die  14  to pads (not visible) on a chip substrate  12 . As shown, a typical current path  18  flows from circuitry (not visible) on the substrate  12 , through a bump  16   a , through circuitry (not visible) on the die  14 , and finally from the die  14  through another bump  16   b  and into other circuitry (not visible) on the substrate  12 . A bump  16  is the element in the current flow path  18  that is often the most susceptible to electromigration damage due to its material, typically a solder, and the fact that the current flow must change directions. 
     As shown in more detail in  FIG. 2 , current flowing through the trace  20  and pad  22  within the die  14  must change direction in order to flow through an opening  25 , through the conductive pad-to-bump interface (referred to hereinafter as the UBM)  26 , through the bump  16  itself, and finally into the substrate pad  28 . As indicated with dotted arrows  15  in  FIG. 2 , this turning causes the current to “crowd” at the upstream side of the bump  16 , resulting in a higher current density, J, in the location of crowding. The mean time to fail (MTTF) under electromigration conditions is generally approximated to be 
     
       
         
           
             MTTF 
             ∝ 
             
               A 
               
                 J 
                 n 
               
             
           
         
       
     
     where A incorporates the effects of temperature and other factors and the power n is in the range of 1 to 2 for lead solders. High local values of the current density, J, may cause failures that are premature in time when compared with the failures that occur when the current is uniformly distributed in the bump  16 . 
     The amelioration of electromigration in bump interconnects is the subject of much study. One prior art solution includes the use of a “bus” structure for high current bumps in order to limit the routable regions within the metal layer(s) used for the bus. 
     The cross-sectional area of a bump affects the rate of electromigration in the bump. Bump cross-sectional area is partially dictated by the bump-to-bump spacing, with higher spacing typically permitting greater cross-sectional area of the bumps. However, with the competition for smaller and faster packaging, the trend has been towards shrinking the bump-to-bump spacing. Thus, future bumps may have smaller cross-sections, leading to the problem of higher current densities in the bumps. 
     The choice of material used to implement the bump can also play a significant factor in the electromigration properties of the bump. Presently, bump material is typically made of either a 90% Pb (lead) solder that is known to exhibit some electromigration resistance or a lead-tin eutectic solder that has significantly less resistance to electromigration damage. Future designs may use lead-free materials which have unknown electromigration issues. The ability to remove the electromigration design restrictions as materials change could be an important design asset. 
     Present designs employ multiple bumps for high current circuits. More electromigration resistant designs may enhance present configurations by carrying these high currents in fewer bumps, thereby reducing chip size and cost or by freeing up bumps for other functions. Future designs could also enjoy these benefits. These advantages may also be shared by lower current signal bumps where, for example, traces may be made narrower which would result in routing enhancements. 
     In view of the foregoing, it would be desirable to have a technique for redistributing the current flow through integrated circuit component connection joints such as flip-chip bumps in order to reduce electromigration damage caused by current crowding in one area of the joint. 
     SUMMARY OF THE INVENTION 
     The present invention is technique for redistributing current flowing to a pad from a trace of an integrated circuit in order to reduce electromigration damage to integrated circuit connection joints connected to the pad, such as flip-chip bumps, caused by current crowding in one area of the joint. The invention is the design and implementation of additional trace routing between a pad of an integrated circuit and the trace delivering current to the pad. The additional trace routing includes an outer trace channel connected to the trace and a plurality of conductive trace leads connecting the outer trace channel to the pad. The original trace is coupled to the pad only through the intervening outer trace channel and conductive trace leads. Thus, all current delivered to the pad flows through the plurality of conductive trace leads. The redistribution of current flow to the pad from a single point of entry (i.e., by a direction connection of the current delivering trace) as in the prior art to multiple points of entry through the plurality of conductive trace leads as implemented according to the invention reduces current crowding in the joint connected to the pad. Preferably, each of the conductive trace leads connecting the outer trace channel to the pad is characterized by a respective impedance that results in a reasonably uniform current density on the pad, which and therefore at the interface to, and through, the connection joint (e.g., flip-chip bump). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a cross-sectional side view of an integrated circuit in a flip-chip package; 
         FIG. 2  is a cross-sectional side view of a portion of a flip-chip assembly illustrating a single solder bump; 
         FIG. 3A  is a cross-sectional side view of the components included in a single bump junction of the flip-chip assembly of  FIG. 1 ; 
         FIG. 3B  is a cross-sectional front view of the bump junction of  FIG. 3A ; 
         FIG. 3C  is an isometric view of the bump junction of  FIG. 3A ; 
         FIG. 3D  is a top plan view of the bump junction of  FIG. 3A ; 
         FIG. 3E  is a perspective view of the trace and pad of  FIGS. 3A-3D ; 
         FIG. 4A  is an isometric view of the components included in a single bump junction of a flip-chip assembly as implemented in accordance with the invention; 
         FIG. 4B  is a top plan view of the bump junction of  FIG. 4B ; 
         FIG. 4C  is a perspective view of the trace and pad of  FIGS. 4A-4B ; 
         FIG. 5  is a pair of graphs illustrating the current density within a bump at the junction with the UBM of a prior art flip-chip assembly and at the same location when implemented in accordance with the invention; 
         FIGS. 6A through 6D  are plan views of alternative embodiments of the invention of pads and associated trace routes that seek to produce a radially uniform inflow of current into the pad and joint attached thereto; 
         FIG. 7  is an operational flowchart of a method for determining trace widths of conductive leads connecting the outer channel to the inner pad of a pad implemented according to the invention; and 
         FIG. 8  is a plan view of an example pad of the invention partitioned into trace segments with associated dimensions for use in determining the trace widths of the conductive leads. 
     
    
    
     DETAILED DESCRIPTION 
     A novel trace routing design for integrated circuit I/O pads is described in detail below that seeks to introduce current flow delivered by a trace into an integrated circuit pad by routing current flow from the trace delivering the current to the pad through a plurality of traces to the pad or through an intermediate trace channel and multiple conductive leads to the pad. Current is therefore introduced to the pad from a number of different paths rather than the single path that results from a direct connection between the trace and pad. A relatively uniform current distribution may be achieved at the pad opening to the bump (i.e., at the conductive junction between the pad and the UBM), and hence at the bump, through selection of the number, pattern, and relative impedances of the conductive leads connecting the outer channel to the pad, resulting in reduced current crowding and reduced electromigration damage in the joint (e.g., flip-chip bump) connected to the pad. For purposes of comparison, the configuration of a traditional prior art solder bump in a flip-chip assembly is shown in  FIGS. 3A ,  3 B,  3 C,  3 D,  3 E, and  3 F. More particularly,  FIG. 3A  is a cross-sectional side view,  FIG. 3B  is a cross-sectional front view,  FIG. 3C  is an isometric view, and  FIG. 3D  is a top plan view of the components included in a single bump junction of the flip-chip assembly  10  of  FIG. 1 .  FIG. 3E  is a perspective view of the trace  20  and pad  22  of  FIGS. 3A-3D . As illustrated in  FIGS. 3A ,  3 B,  3 C, and  3 D, the trace  20  is conductively connected to the pad  22  on the outermost trace layer of the integrated circuit  14  (of  FIG. 1 ). The pad  22  is capped with a passivation layer  24 , typically comprising either a nitride or a polymer. An opening  25  is etched into the passivation layer  24  and the UBM  26  is plated over both the opening  25  and a portion of the passivation layer  24 . Solder attaches to the UBM  26  during the bumping process to form bump  16 , which conductively connects the UBM  26  and the substrate pad  28  when the die is flipped and attached to the substrate  12 . The substrate pad  28  is connected to substrate via  30  for routing to circuitry implemented on or otherwise connected to the substrate  12 . The metal layers M 1 , . . . , Mn, vias, and UBM are implemented using highly conductive material, typically copper, gold, or other elements or compounds of high conductivity. The dielectric layers D 1 , . . . , Dn- 1  and  48  are typically implemented using a polymer such as Benzoclyclobutene (BCB). The bump  16  material is typically copper or a lead solder compound such as PbSn, AuPb, PbAg, etc. 
     In the traditional configuration, as shown in  FIG. 3D , current enters the pad  22  from the trace  20  along the path  18 , and, as illustrated in  FIG. 2 , causes the greatest current densities in the solder bump  16  in the area indicated at  15  near the opening  25  closest to the trace  22 . 
     In a design implemented according to the present invention, as shown in  FIGS. 4A ,  4 B, and  4 C, rather than connecting directly to the pad  22 , the trace  20  is instead connected to an outer trace channel  102 , which is connected to multiple conductive trace leads  106  that connect the outer trace channel  102  and the pad  104 . The impedance of the multiple conductive leads  106  may be respectively customized to achieve a relatively uniform current distribution seen on the pad  22 . Impedance tailoring of the various conductive leads is reflected in the various widths of the conductive trace leads. A methodology for determining the widths of the leads is described hereinafter. 
     In the inventive configuration, current delivered by the trace  20  flows into the outer trace channel  102 , as indicated by the dotted arrows  108  illustrated in  FIG. 4B , and is routed through the outer trace channel  102  to and through the plurality of conductive trace leads  106 , and into the pad  104 . As described above, the respective resistances of the plurality of conductive conductive trace leads  106  are preferably implemented by design to distribute equal current flow (within a reasonable margin of error) through each lead  106 , thereby producing a more uniform current density on the pad  104  and in a bump  16  connected to the pad  104 . 
       FIG. 5  presents graphs showing for comparison the current density for a bump  16  coupled to the traditional integrated circuit pad  22  of  FIGS. 3A-3E , and the current density for a bump  16  coupled to a pad coupled to a current delivering trace using the trace routing design of the invention of  FIGS. 4A-4C . As illustrated, the embodiment of the trace routing design of the invention shown in  FIGS. 4A-4C  results in a 40% reduction in overall maximum current density. As also illustrated, in the traditional trace-to-pad configuration, current crowding occurs in the area of the bump  16  closest to connection of the trace  20  to the pad  22  and directly below the portion of the UBM within the opening  25 . In the configuration of the invention, however, the current is distributed substantially equally around the area near the outer circumference of the opening  25  between the pad  104  and the UBM  26  on the bump  16 , resulting in a lower maximum current density across the bump, and therefore eliminating or significantly reducing any current crowding. While the methodology of the invention does not address the intensification of current due to turning effects, a lower value of current density is obtained in the bump  16  due to the use of a plurality of conductive leads  106  to introduce the current to the bump  16  at multiple locations. For the example shown, the invention based design has a current density which is 40% that of the traditional design. For a given current and considering that the exponent, n, may have values between 1 and 2, the invention-based design will have predicted electromigration lives that are 2.5 to 6.25 times greater than that of the traditional configuration. 
       FIGS. 6A through 6D  respectively illustrate alternative illustrative embodiments of trace routing designs implemented according to the principles of the invention.  FIG. 6A  illustrates a trace routing design  110  that includes an outer channel  112  that routes current around half of the inner pad  114  to introduce current flow through two conductive leads  116   a  and  116   b  on opposite sides of the pad.  FIG. 6B  illustrates a trace routing design  120  that includes an outer channel  122  that routes current five-eighths of the way around the pad  124  in one direction and an eighth of the way around the pad  124  in the other direction, and includes four conductive leads  126   a ,  126   b ,  126   c ,  126   d  for introducing current to the pad  124 .  FIG. 6C  illustrates a trace routing design  130  that includes an outer channel  132  that routes current a quarter of the distance around the pad  134  in two different directions, and includes three conductive leads  136   a ,  136   b ,  136   c  for introducing current to the pad  134 .  FIG. 6D  illustrates a trace routing design  140  that includes an outer channel  142  that routes current five-eighths of the way around the pad  144  in one direction and an eighth of the way around the pad  144  in the other direction, and includes four conductive leads  146   a ,  146   b ,  146   c ,  146   d  for introducing current to the pad  144 . 
     As illustrated by these embodiments, the trace routing design is not dependent on the orientation of the inner pad, the junction point of the main current delivering trace connection to the outer channel, the number of conductive leads connecting the outer channel to the inner pad, or the path of the outer channel. Different improvements may be obtained for configurations other than those illustrated. It should be emphasized that symmetry is not a required attribute of the invention, nor is any particular number of conductive leads. Rather, the invention is to introduce the current into the pad from multiple locations such that a reasonably uniform current density is achieved at the bump. In some configurations this will translate to equalizing the current flowing through each of the conductive leads to the pad. In other configurations the currents flow needs to be set up to allow flow in proportions other than equal proportion. Through calculation, the respective impedances of the conductive leads (e.g., through adjusting the widths of the conductive trace leads assuming a constant trace thickness) can be designed and implemented to achieve the desired current density on the pad and therefore at the interface to the bump. 
     It should also be emphasized that the same invention-based design philosophy may be applied, for example, within the pad/via/trace design in the substrate or in integrated circuit components other than the illustrated flip-chip embodiment. 
       FIG. 7  is an operational flowchart illustrating a methodology for determining trace widths of conductive leads connecting the outer channel  102  to the inner pad  104 . For simplicity of illustration, the method illustrated is limited to application of non-branching outer channels (e.g., the trace routing design of  FIG. 6A ) of constant width and thickness. As shown, the method includes the step of obtaining the trace thickness, for each conductive lead to be implemented, obtaining the length and width of the outer channel from the trace junction of the pad  100  to the junction of the conductive lead (step  201 ). The width of one of the conductive leads is selected or obtained (step  202 ). For each remaining conductive lead (determined in step  203 ), one of the remaining conductive leads is selected (step  204 ), and the width of the selected remaining conductive lead is calculated such that the ratio of the width of the selected remaining conductive lead to the length of the selected remaining conductive lead (obtained in step  201 ) is substantially equal to the ratio of the known width of the first conductive lead to the known length of the first conductive lead ( 205 ). The calculation for branching traces (e.g., the trace routing designs of  FIGS. 4A through 4C  and  6 B through  6 D) or those with non-constant geometry follows basic circuit theory where the resistances for each path are tailored such that the current flowing into the pad is the same for all branches. This calculation will be obvious to those skilled in simple resistive circuit theory. For example, consider the two-conductive lead trace design  110  of  FIG. 6A . The goal is to construct equal resistance paths from the trace  20  to the pad  114  through the outer channel  112  and each of the conductive leads  116   a  and  116   b . By definition, the resistance R of a conductor is defined as R=ρ*L/w*t, where ρ is resistivity, L is trace length, w is the trace width, and t is the trace thickness.  FIG. 8  illustrates the two-conductive lead trace design  110  where the outer channel  112  is partitioned into trace segments. Suppose that the width w, thickness t, and resistivity ρ of the channel  112  are all constant. Then, suppose the width of conductive lead  116   b  is selected to be 10 um. The calculation may thus be stated as: Given w 116b =10, find w 116a . So, w 116b /L 116b =w 116a /L 116a , or w 116a =L 116a * (w 116b /L 112a +L 112b +L 112c +L 116b )=10*10/(60+130+65+10)=0.377 um. 
     Analysis and comparison of traditional and invention-based trace routing designs in determining the current density distribution within the pad and bump and, in particular, at the interface with the UBM  26  shows that a design implemented according to the principles of the invention has significantly lower current densities at the critical UBM location than those in the traditional design. The maximum current densities are taken to be metrics for the electromigration life of the bumps in each configuration. 
     While the illustrative embodiments of the invention as presented herein address the metal traces within the die, the invention is also applicable to other electrical designs, for example, the substrate traces, where the combination of current levels, changes in current direction and material sensitivity lead to electromigration problems. 
     Although this preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. It is also possible that other benefits or uses of the currently disclosed invention will become apparent over time.

Technology Category: 5