Abstract:
A solder mask defined bond pad or a non-solder mask defined bond pad may be configured to center the solder over the bond pad using either surface attractive forces or capillary action. In some embodiments, a stub trace may be provided, for example, in opposition to the real trace to provide a capillary counter-attractive force on the solder. In other embodiments, the surface attractive action of the edge of the solder mask may be utilized to center the solder. In still other embodiments, the natural attractive force of a trace on solder may be utilized to appropriately position solder where desired, for example, to line up with other solder deposits.

Description:
BACKGROUND 
     This invention relates generally to solder bonding techniques for integrated circuit devices. 
     Referring to  FIG. 10 , commonly solder is deposited on a solder pad  62  which is coupled to other electrical components on an integrated circuit by a trace  60 . The solder deposition area is defined by the inwardmost edge  64  of a solder mask. Thus, in the embodiment illustrated in  FIG. 10 , the solder is deposited inside the circle  64 . The solder mask prevents solder outflow over the mask thereby preventing the solder from moving outwardly beyond the edge  64 . 
     The solder may be in the form of conventional solder balls which are deposited in a solid configuration and then reflowed thereafter. Alternating the solder may be a liquid or paste upon deposition. 
     Referring to  FIG. 11 , one problem with existing techniques for depositing solder is that when soft the solder  66  tends to wick along the trace  60 . Without limitation, it is believed that the wicking is a result of capillary attraction between the solder  66  and the trace  60 . As a result, the solder  66  ends up being displaced with respect to the pad  62 , as indicated in  FIG. 11 . In particular, the solder may abut the solder mask edge  64 . Generally, the solder does not extend onto the solder mask since the mask functions to control solder flow. 
     Thus, improper contact may result between the solder and the solder pad  62  as a result of the wicking action of the solder. Of course, this problem may be reduced by decreasing the diameter of the opening  64  in the solder mask. However, this creates tighter tolerances in the process flow. One adverse result may be that the solder mask opening is misaligned to the pad  62  to such an extent that the solder mask opening does not permit the solder to be placed on the pad. 
     In ball grid array (BGA) packaging techniques an array of solder pads may be aligned with an array of solder balls. If the balls tend to wick away from their solder pads, the balls may become misaligned with other balls in the array. Thus, there may be no way to cause an integrated circuit connector to appropriately connect to all the balls because all the balls have been randomly misaligned. Referring to  FIG. 12 , the ball  68  on the top has wicked to the right because its trace  60  extends to the right whereas the ball  76  on the bottom has wicked to the left because of the leftward extension of its trace  70 . The center line “CL” of the pads  62  and  72  may have been the projected alignment between the balls. In fact the balls are substantially misaligned. 
     Still another problem that may arise in the prior art is the surface action effects of the edge of the solder resist mask.  FIG. 13  illustrates a conventional solder mask defined pad (SDP). In this case, the useful portion of the pad  80  is effectively defined by the opening  82  in the solder mask. This is because the size of the opening  82  is less than the size of the pad  80 . Thus, wicking along the trace  78  may be prevented. However, the mask may tend to attract the solder  84  to its edge, for example as a result of surface attraction effects. Again, the problem is similar to the problem described previously in that the solder tends to be attracted away from its desired location. 
     Thus, there is a need for better ways to appropriately position solder on bond pads coupled to conductive traces. 
     SUMMARY 
     In accordance with one embodiment, a bond pad assembly may include a bond pad and a trace coupled to the pad. The trace extends away from the pad in a first direction. A trace stub is coupled to the pad and extends away from the pad in a direction other than the first direction. 
     Other aspects are set forth in the ensuing detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an enlarged top plan view of one embodiment to the present invention; 
         FIG. 2  is a cross-sectional view taken generally along the line  2 — 2  shown in  FIG. 1  after solder has been placed on the pad; 
         FIG. 3  is an enlarged top plan view of another embodiment of the present invention; 
         FIG. 4  is a cross-sectional view taken generally along the line  4 — 4  shown in  FIG. 3  after solder has been placed on the pad; 
         FIG. 5  is an enlarged top plan view of still another embodiment to the present invention; 
         FIG. 6  is a cross-sectional view taken generally along the line  6 — 6  in  FIG. 5 ; 
         FIG. 7  is an enlarged top plan view of still another embodiment of the present invention; 
         FIG. 8  is a cross-sectional view taken generally along the line  8 — 8  in  FIG. 7  after the solder has been placed on the pads; 
         FIG. 9  is an enlarged top plan view of another embodiment of the present invention; 
         FIG. 10  is an enlarged top plan view of an embodiment in accordance with the prior art; 
         FIG. 11  is an enlarged top plan view of another embodiment in accordance with the prior art; 
         FIG. 12  is an enlarged top plan view of still another embodiment in accordance with the prior art; and 
         FIG. 13  is an enlarged top plan view of still another embodiment in accordance with the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a bond assembly  10  includes a bond pad  14  coupled to a trace  12  extending away from the bond pad  14  in a first direction. The bond assembly may be formed on a support which may be, for example, an integrated circuit die, an interposer, or a printed circuit board. While the bond pad  14  is illustrated as being circular other shapes can be used as well. The bond pad  14  may be utilized in connection with packaging a variety of different integrated circuit devices. 
     In one application, the bond pad  14  may be arranged to interact with solder balls to implement a flip chip bonding technique, a ball grid array bonding technique or any of the variations of bump-type interconnections which may be known to those skilled in the art. In ball grid array packaging techniques, a relatively solid ball is positioned on the bond pad and subsequently reflowed. In other techniques, liquid or semi-liquid solder may be utilized which may flow upon deposition without the application of heat. 
     A trace stub  16  extends away from the bond pad  12  in a second direction. Advantageously, the stub  16  may be made of the same material and may be of the same width and thickness as the trace  12 . The first and second directions may be diametrically opposed. 
     The region which may receive the solder may be greater than the size of the bond pad  14 . Conveniently, the potential solder receiving area may be defined by a solder mask whose inward extent is marked by the solder mask edge  18 . Thus, solder is masked away from the remainder of the device with the exception of the area inside the edge  18 . 
     Referring to  FIG. 2 , a solder ball  20  has been reflowed over the pad  14 . As shown in  FIG. 2 , the solder mask edge  18  actually overlaps the trace stub  16 . This provides greater tolerances and ensures that the stub  16  will extend beyond the solder mask edge  18 . With this configuration, if the solder attempts to wick to the left to follow the trace  12  due to capillary action or any other reason, it will be pulled back to the right by the action of the stub  16 . Thus, the forces applied by the stub  16  counteract the wicking action of the trace  12 . In some embodiments, it may be desirable to make the solder pad  14  relatively small so that the solder ball  20  is acted upon simultaneously by both the trace  12  and stub  16 . 
     In another embodiment of the present invention, shown in  FIG. 3 , a bond assembly  20  includes an enlarged elliptical or teardrop-shaped bond pad portion  24  which is designed to reduce the capacitance caused by the bond pad main circular section  26 . Thus, the bond pad portion  24  has a elliptical configuration of smaller size than that the main circular section  26 . The portion  24  is coupled to the trace  22  on one end. In the embodiment illustrated in  FIG. 3 , a matching or mirror image portion or stub  28  is formed on the other side of the section  26 . The function of the stub  28  is to counteract any wicking action resulting from the portion  24 . In some embodiments an additional stub, like the stub  16 , may be caused to extend outwardly from the stub  28  in opposition to the trace  22 . 
     In the embodiment illustrated in  FIG. 3 , the matching stub  28  does not extend outside of the boundary defined by the solder mask edge  18 . Thus, in some embodiments it may be preferable to cause the matching portion to extend beyond the solder mask edge and in other cases this may not be desirable. 
     Referring to  FIG. 4 , when a solder ball  20  is positioned on the section  26 , it is equally attracted to the left and to the right by the opposed portions  24  and  28 . Thus, the solder ball  20  may center on the section  26 . 
     The embodiment in  FIG. 1  illustrates a non-solder mask defined pad (NSDP). However, as explained in more detail hereinafter, the present invention is also applicable to solder mask defined pads (SDP). Referring now to  FIG. 5 , showing an SDP embodiment of the present invention, a solder mask has a cloverleaf-shaped edge  32  which extends inwardly of the bond pad  34  and its trace  30 . Each of the lobes  33  of the cloverleaf-shaped edge  32  may have a surface action attraction on the solder ball  36 . 
     By providing four sets of identically shaped clover leaf shaped lobes, the action of the edges  32  on the solder may be neutralized. One force on the solder is believed to be due to surface tension effects. Moreover, by having the convex edges  35  of the solder mask  32  substantially spaced apart by a diameter approximately equally the diameter of the solder  36 , the solder tends to be maintained substantially centrally, as illustrated in  FIG. 6 . 
     Referring now to  FIGS. 7 and 8 , an embodiment in which the wicking action of a traces  40  and  46  may be used to achieve a desired orientation for solder balls  20  and  20 A is illustrated in an NSDP arrangement. In this case, the bond pads  42  and  50  may be placed relatively closer together than is normally the case. This may be done by causing the bond pad  50  to overlap with the trace  40  coupled to the bond pad  42  so that a nested configuration may be achieved. In each case, a solder mask edge  44  or  48  is defined which delimits the extent to which the solder ball  20  or  20   a  may move. 
     After being deposited on the pad  42  and reflowed, the solder ball  20  may tend to move to the left due to the wicking action of the trace  40 . Similarly, when the solder ball  20   a  is placed on the pad  50 , it tends to wick to the right. As a result of the wicking action, the solder balls  20  and  20   a  line up one above the other exactly as desired. Thus, in this case, the adverse effect of trace wicking is used to obtain the desired alignment between the balls. The desired ball alignment may be useful in causing the balls to interact with other contacts on another device. In some cases, this technique may enable the bond pads to be nested and thereby packed together more closely. 
     Turning now to  FIG. 9 , still another embodiment of a non-solder mask defined pad is illustrated. In this case, the pad  102  is coupled to a trace  100 . A solder receiving area is defined by the edge  110  of the solder mask. A trace stub  104  is provided as illustrated previously in connection with  FIG. 1 . In addition, a pair of trace stubs  108  and  106  extend transversely to the lengths of the trace  100  and the stub  104 . The stubs  106  and  108  center the solder (not shown) along the axis transverse to the axis of the trace  100  and the stub  104 . The stubs  106  and  108  provide effectively vertical centering in the orientation shown in  FIG. 9 , while the stub  104  together with the trace  100  provide horizontal centering. Thus, the embodiment shown in  FIG. 9  prevents the solder from moving up or down. The solder may move up and down, not because of the wicking action of the trace, but for some other reason such as other attractive forces, or tilting of the pad supporting surface. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.