Abstract:
A solder ball pad is provided for mounting and connecting of electronic devices and, more particularly, apparatus and methods are disclosed providing an improved solder ball pad structure on a substrate, such as a printed circuit board (“PCB”) or a semiconductor die, while enabling better use of the spaces between adjacent solder ball pads, and at the same time providing increased surface area for bonding to a solder ball. More particularly, the inventive solder ball pad structure comprises a terminal pad exposed through an aperture in an insulative mask having a bond pad layer comprising at least another metal layer formed over, at most, a portion of the exposed portion of the terminal pad. Methods of manufacture and substrates incorporating same are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of application Ser. No. 10/230,962, filed Aug. 29, 2002, now U.S. Pat. No. 6,762,503. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the mounting and connecting of electronic devices and, more particularly, to apparatus and methods providing an improved solder ball pad structure on a substrate such as a printed circuit board (“PCB”) or a semiconductor die. 
   2. State of the Art 
   An increasing demand for electronic equipment that is smaller, lighter, and more compact has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting areas or “footprints.” 
   One response to this demand has been the development of the so-called “flip-chip” method of attachment and connection of semiconductor chips to substrates. Sometimes referred to as the “Controlled Collapse Chip Connection,” or “C4,” method, the technique involves forming balls of a conductive metal, e.g., solder or gold, on input/output connection pads on the active surface of the chip, then inverting, or “flipping” the chip upside down, and “reflowing” the conductive balls, i.e., heating them to the melting point, to fuse them to corresponding connection pads on a substrate. 
   Another response has been the development of a so-called ball grid array (“BGA”) semiconductor package that “surface mounts” and electrically connects to an associated carrier substrate, e.g., a printed circuit board (“PCB”), with a plurality of solder balls in a method sometimes referred to as the “C5” method that is analogous to the flip-chip method described above for mounting and connecting dice. 
   In both the C4 die and C5 package mounting and connection methods, a plurality of solder balls is attached to respective solder ball mounting lands, or pads, defined on a surface of the die or interposer substrate. The solder ball mounting pad may be defined by an opening in an insulative layer or mask called a “passivation layer” in the case of a semiconductor die, or a “solder mask” in the case of an interposer substrate of a BGA package, as described below. The interposer substrate in a BGA package may comprise a rigid or flexible sheet material. 
   In a solder-mask-defined (“SMD”) solder ball pad, an aperture formed in the mask over a terminal pad defines the solder ball pad mounting area. Typically, the terminal pad comprises a layer of metal, e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or a multilayer combination of the aforementioned materials that has been laminate and/or plated on a surface of the substrate sheet and then patterned using known photolithography techniques. Further, one or more circuit traces may be formed simultaneously with the terminal pads using the same processes. In addition, a plated through-hole, called a “via,” may also be formed and may connect the pad layer with the opposite surface of the substrate sheet. 
   A solder mask is then formed over the metal terminal pad and may comprise an acrylic or a polyimide plastic or, alternatively, an epoxy resin that is silk screened, spin-coated or applied as a preformed film on the substrate sheet. An aperture is formed in the solder mask to expose a portion of the terminal pad, but not any portion of the surrounding substrate surface. A solder ball may be attached to or formed on the terminal pad area thus exposed; however, the solder mask prevents the solder of the solder ball from attaching to any portion of the terminal pad other than the mounting area that is exposed through the aperture. Thus, the exposed area is referred to as an SMD-type of solder ball mounting pad. 
   Comparatively, a nonsolder-mask-defined (“NSMD”) solder ball mounting pad may be formed in a similar manner, the exception being the size of the aperture in the solder mask. In particular, typically, the NSMD pad exposes the entire terminal pad, at least a portion of the surface of the substrate sheet and, optionally, a portion of an adjacent circuit trace, such that the molten solder of the solder ball can attach to the entire surface and peripheral vertical side surface of the terminal pad thus exposed. Typically, a circular-shaped terminal pad and a portion of a circuit trace are exposed in an NSMD solder ball mounting pad arrangement. The connection area of both the SMD-type and NSMD-type solder ball mounting pads may be coated with a nickel layer and then a gold layer to enhance wettability of solder thereon. 
   Each of the conventional SMD and the NSMD solder ball mounting pads have some advantages as well as disadvantages associated with it. 
   Turning to the SMD solder ball pad, it provides relatively good “end-of-line” (i.e., at the end of the semiconductor package fabrication line) ball shear resistance because the solder mask overlaps the peripheral edge of the terminal pad proximate to the exposed area defining the solder ball mounting pad and, therefore, resists ripping of the terminal pad from the substrate when mechanical forces act on the solder ball attached thereto. In contrast, the NSMD solder ball pad has a relatively lower end-of-life shear resistance because the solder mask does not cover the peripheral edge of the NSMD terminal pad. 
   The SMD solder ball pad also affords relatively better control of the lateral (x-y) position of the solder ball on the surface of the substrate than does an NSMD solder ball pad. This is because the lateral position of the solder ball on the substrate may be affected by two factors: 1) the position on the substrate of the centroid of the aperture in the solder mask, if the vertical wall of the aperture interacts (e.g., touches, or electrostatically interacts) with the solder ball, and 2) the position of the centroid of the area of the metal pad layer that is exposed by the opening in the mask, i.e., the area wetted by the molten solder of the solder ball when the latter is attached to the solder ball pad. In both instances, the center of gravity of the solder ball tends to align itself over each of the two respective centroids if both factors apply. As a result, when the centroid of the aperture does not coincide with the centroid of the exposed area of the mounting pad and the vertical wall of the aperture interacts with (e.g., touches) the solder ball, the center of gravity of the solder ball may be positioned approximately half way along a line extending between the two centroids. Since in an SMD solder ball pad the aperture in the solder mask exposes only pad layer metal, the centroid of the aperture and exposed metal pad layer coincide. Thus, so long as the aperture in the solder mask is located within the periphery of the metal pad layer, the lateral tolerances of the SMD solder ball will depend substantially on the lateral positional tolerances on the centroid of the aperture. 
   However, the shape of the NSMD solder ball pad exposed by the aperture in the solder mask includes a terminal pad portion as well as a portion of the circuit trace. Further, the vertical wall of the aperture may not touch the solder ball. Consequently, the centroid of the NSMD solder ball pad, i.e., of the exposed area of metal, is shifted slightly toward the circuit trace and away from the centroid of the opening, which is typically centered on the terminal pad portion. Hence, the center of gravity of the solder ball will be positioned according to the respective centroids of the NSMD solder ball pad and the circuit trace. Thus, the lateral tolerances on the solder ball on an NSMD solder ball pad may depend not only on the lateral tolerances of the centroid of the aperture, but also the lateral tolerances of the centroid of the exposed metal of the metal pad layer as well. Moreover, even without the presence of a circuit trace, misalignment of the solder ball can still occur in an NSMD pad if the centroid of the exposed pad is not sufficiently aligned with the centroid of the aperture, and thus a vertical sidewall of the aperture interacts with the solder ball. 
   While the lateral misalignment of a solder ball relative to an opening resulting from this “shift” is relatively small, it should be understood that a C4-mounted die or a C5-mounted semiconductor package can typically have a large number, e.g., up to nine hundred, of such solder balls on its mounting surface, and that accordingly, these slight misalignments in the array of balls can be additive, such that in some cases, the die or package cannot be successfully mounted to an associated mounting surface. 
   As a further comparison between SMD and NSMD solder ball pads, the solder ball attached to an NSMD solder ball pad attaches to the vertical side surface of the exposed metal of the terminal pad including the circuit trace(s), if any. It is postulated that this side surface attachment and resulting arcuate attachment structure helps to distribute stresses resulting from thermal aging so that the stresses do not concentrate at the interface between the NSMD solder ball pad and the solder ball. Thus, the NSMD may provide an improved resistance to thermal stresses over the SMD solder ball pad, the solder ball/pad interface of which consists of a simple planar interface between the exposed portion of the terminal pad and the solder ball. 
   U.S. Pat. No. 6,201,305 to Darveaux et al., as well as U.S. Pat. No. 5,872,399 to Lee, each describes a solder ball pad structure. More specifically, the Darveaux reference describes an NSMD-type solder ball pad structure wherein a layer of metal on the substrate is formed into a terminal pad, the pad having at least two spokes radiating outwardly therefrom. The pad structure with spokes is exposed by way of an aperture formed through the solder mask such that the terminal pad and an inner portion of each of the spokes is exposed therethrough, and an outer portion of each of the spokes is covered by the mask. The Lee reference describes a solder ball pad structure having a terminal etching hole as well as a plurality of etching holes at the outer portion of the solder ball pad structure for increasing the contact area for a solder ball. 
   Another area of interest is the design flexibility in the number of circuit traces that may be operably positioned to run between two adjacent solder ball pads with adequate spacing between the traces and between the traces and the solder ball pads. More specifically, the aforementioned tolerance considerations, as well as the differences in the formation of SMD and NSMD solder ball pads, must be factored in determining the spacing between circuit traces and solder ball pads. Of course, dimensional tolerances, as well as parameters required to achieve a robust design, limit the ability to position addition circuit traces between solder ball pads for a given solder ball pad design pitch. 
   In view of the foregoing, a method for fabricating solder ball mounting pads on a substrate and resulting solder ball mounting pads which improve on both types of conventional solder ball pads and eliminate some of their respective disadvantages would be desirable. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention comprises an apparatus and method providing an inventive solder ball pad structure and substrates, electronic device assemblies and systems employing same. The solder ball pad structure of the present invention includes a metal terminal pad that is partially exposed by an aperture in a solder mask layer which does not expose any portion of a surrounding substrate surface. In addition, an optional metallic or conductive polymer interface layer may be formed onto at least a portion of the exposed area of the metal terminal pad and onto at least a portion of the vertical sidewall of the solder mask defining the aperture as well as extending onto the surrounding top horizontal surface of the solder mask. Alternatively, an optional nonconductive polymer interface layer may be formed onto the surrounding top surface of the solder mask. A copper layer comprising an electroless copper seed layer as well as an optional electroplated copper layer may be formed over the solder mask, into the aperture and over the exposed portion of the terminal pad. Nickel and gold layers may be applied by electroplating to the copper layer to enhance the wettability to solder of the resulting pad surface. The solder ball pad structure of the present invention may be termed a ball pad on solder mask or “BPS.” 
   The solder ball pad structure of the present invention may provide a variety of advantages. First, the lateral positional tolerance of an attached solder ball is largely determined by the tolerances associated with the formation of the solder mask, similar to the SMD solder ball pad. Additionally, the optional interface layer and subsequent metal layers which may be attached to the metal terminal pad enable the solder ball to attach to the vertical side surface of the aperture in the resulting structure, which configuration may provide enhanced thermal stress distribution in the solder ball connection. Also, the solder ball pad of the present invention provides increased surface area for solder ball attachment, as well as an indented surface for attachment which may further strengthen the bond between a solder ball and the inventive solder ball pad structure. 
   As another advantage, the solder ball pad structure of the present invention provides additional, usable lateral space on the substrate between adjacent solder ball pads for circuit traces and circuit trace spacing. Because the layered structure above the metal terminal pad on the substrate increases the area for attachment for the solder ball, in excess of the metal pad layer area that is exposed via the solder mask aperture, the size of the metal terminal pad may be reduced. This enables additional circuit traces or spacing to be employed between adjacent solder ball pads, as desired. 
   It is also contemplated that the structure of the present invention has utility with discrete conductive elements other than solder, such as conductive or conductor-filled epoxy. Accordingly, the term “solder ball pad” is exemplary and not limiting of the scope of the present invention. 
   Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention: 
       FIG. 1A  is a top view of a conventional SMD solder ball pad; 
       FIG. 1B  is side cross-sectional view of the SMD solder ball pad shown in  FIG. 1A ; 
       FIG. 2A  is a top view of a conventional NSMD solder ball pad; 
       FIG. 2B  is side cross-sectional view of the NSMD solder ball pad shown in  FIG. 2A ; 
       FIG. 3A  is a top view of an embodiment of the solder ball pad structure of the present invention; 
       FIG. 3B  is a side cross-sectional view of an embodiment of the solder ball pad structure shown in  FIG. 3A ; 
       FIG. 4A  is a top view of an SMD solder ball pad configuration wherein two circuit traces extend between two SMD solder ball mounting pads; 
       FIG. 4B  is a side cross-sectional view of the SMD solder ball pad configuration shown in  FIG. 4A ; 
       FIG. 5A  a top view of a solder ball pad configuration of the present invention wherein three circuit traces extend between two solder ball mounting pads; 
       FIG. 5B  is a side cross-sectional view of the solder ball pad configuration of the present invention shown in  FIG. 5A ; 
       FIG. 6A  is a top view of an NSMD solder ball pad configuration wherein two circuit traces extend between two NSMD solder ball mounting pads; 
       FIG. 6B  is a side cross-sectional view of the NSMD solder ball pad configuration shown in  FIG. 6A ; 
       FIGS. 7A through 7L  show top views and associated side cross-sectional views of different embodiments of the present invention; 
       FIGS. 8A and 8B  depict an exemplary process flow for forming the solder ball pad structure of the present invention using a conductive polymer interface layer; 
       FIG. 9  depicts placement of a nonconductive polymer interface layer in accordance with the present invention; 
       FIGS. 10A through 10D  depict a first exemplary process flow for forming the solder ball pad structure of the present invention using an electrolessly plated interface layer; and 
       FIGS. 11A through 11D  depict a second exemplary process flow for forming the solder ball pad structure of the present invention using an electrolessly plated interface layer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring again to conventional practices to provide a more detailed basis for comparison with the present invention, and not in any way to limit the scope thereof,  FIG. 1A  is a top view of a portion of a conventional SMD substrate  10  having a solder-mask-defined (“SMD”) solder ball mounting pad  28  formed thereon.  FIG. 1B  is a cross-sectional view looking into the SMD substrate  10  and mounting pad  28  along the lines IB—IB in FIG.  1 A. The SMD substrate  10  may comprise a sheet  12  of an insulative material, such as bismaleimide triazine, flexible polymide film or tape, fiberglass, polyimide tape, ceramic, or silicon, or, alternatively the SMD substrate  10  may comprise a semiconductor chip or die. The SMD substrate  10  typically comprises a layer of metal, e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or a combination of the foregoing that has been laminated and/or plated on a surface of the insulative sheet  12 , then patterned using known photolithography techniques into a terminal pad  14 , which may include one or more circuit traces  16  (shown by dotted lines) extending therefrom. In addition to the circuit traces  16 , a plated through-hole, called a “via” (not shown), may connect the terminal pad  14  with the opposite surface of the insulative sheet  12  as known in the art. 
   An insulative layer in the form of solder mask  20  is formed over the metal layer, including the terminal pad  14 . The solder mask  20  may comprise an acrylic or polyimide plastic or, alternatively, an epoxy rosin that is silk screened or spin-coated on the insulative sheet  12 . A dry film solder mask may also be employed. An aperture  19  is formed in the solder mask  20  to expose a mounting pad  28  of the terminal pad  14 , and a solder ball  24  (shown in dotted outline in  FIG. 1A ) is attached to, or formed on, the mounting pad  28  thus exposed. Since the solder mask  20  prevents the solder of the solder ball  24  from attaching to any portion of the terminal pad  14  other than the mounting pad  28  that is exposed through the aperture  19 , the mounting pad  28  is referred to as a solder-mask-defined or SMD-type of solder ball mounting pad, as described above. 
   In further illustration of conventional practices for purposes of comparison with the present invention and not in any way in limitation of the scope thereof, a conventional NSMD substrate  11  is illustrated in the top view of  FIG. 2A , wherein features similar to those in the SMD substrate  10  of  FIG. 1A  are numbered similarly.  FIG. 2B  is a cross-sectional view looking into the NSMD substrate  11  and mounting pad  28 ′ along the section lines IIB—IIB in FIG.  2 A. 
   As may be seen from a comparison of the two sets of figures, the respective mounting pads  28  and  28 ′ are very similar, the exception being the relative size of the apertures  19  and  19 ′ in the solder mask  20 . In particular, in the NSMD mounting pad  28 ′ of  FIGS. 2A and 2B , the aperture  19 ′ exposes the entire terminal pad  14 , along with a portion of the surface of the insulative sheet  12  and a portion of the optional, adjacent circuit trace  16 , such that the molten solder of the solder ball  24  can wet and attach to not only the entire upper surface of the terminal pad  14 , but also to the vertical peripheral side surface  26  of the terminal pad  14  and the optional circuit trace  16 . Along the vertical peripheral side surface  26  of the terminal pad  14 , the solder ball  24  attaches and forms a curved attachment surface  29  with the vertical peripheral side surface  26  of the terminal pad  14 . 
   It is conventional in the industry to plate solder ball mounting pads  28  and  28 ′ with a layer of nickel, followed by a layer of gold, shown in combination in  FIGS. 1B and 2B  as solderability enhancement layer  18 , to improve the solderability of the pads. Alternatively, terminal pads  14  may be supplied with nickel/gold, tin/lead, or silver coatings, or may be treated to prevent oxidation of the metal surface of the terminal pad  14 . 
     FIGS. 3A and 3B  show a top and side cross-sectional view of the BPS substrate  40  with solder ball mounting pad  36  according to the present invention. Solder mask  20  exposes an area of the terminal pad  41  on insulative sheet  12  by way of aperture  23 . Interface layer  38  is formed onto the exposed surface area of the terminal pad  41  as well as extending onto the vertical sidewall of the aperture  23  and onto the top horizontal surface of the solder mask  20 . Interface layer  38  may be used to enhance the adhesion of the subsequent copper layer to the solder mask  20  surface, and may comprise an epoxy, such as HYSOL®EO1073 or EO1075, from Henkel Loctite Corporation, Conn. Interface layer  38  may optionally comprise a metal layer formed by using an electroless plating solution or a conductive polymer, as described in more detail below. Copper layer  48  is formed over the terminal pad  41  as well as interface layer  38 , if present, thus extending along the horizontal portion of the terminal pad  41  and onto the sidewall of the solder mask  20  defining aperture  23 , and also onto the horizontal top surface of the solder mask  20 . Copper layer  48  may comprise an electroless copper seed layer (which may be the interface layer  38 ) followed by an electroplated copper layer or may be otherwise formed as known in the art. Further, nickel and gold layers, collectively shown as solderability enhancement layer  18  for clarity, may be applied to the copper layer  48  to enhance the wettability to solder of the resulting mounting pad surface. Nickel is used to prevent diffusion of copper to the solder ball pad surface and gold is used for solder wettability. Thus, optional interface layer  38 , copper layer  48  and solderability enhancement layer  18  together comprise a solder ball pad layer  60 . 
   Because the interface layer  38  as well as the copper layer  48  and solderability enhancement layer  18 , due to their extension up the sidewall of solder mask  20  defining aperture  23  and over onto the outer surface of solder mask  20 , may provide a larger surface area than the area that would be exposed by aperture  19  in a typical SMD-type solder ball pad, the size of terminal pad  41  of a BPS solder ball pad structure may be accordingly reduced. Stated another way, to achieve a final bonding area that is equal to a given SMD mounting pad area, the terminal pad  41  formed from the metal layer deposited on the surface of the insulative sheet  12  may be smaller than the terminal pad  14  of an SMD or NSMD configuration. Reducing the size of terminal pad  41  may allow for more lateral space between adjacent terminal pads to become available on the surface of insulative sheet  12 . By way of example only, solder ball pad layer  60  may exhibit a diameter of about 0.33 millimeters or larger and a total surface area of about 0.05 square millimeters or greater. 
   Moreover, the combination of interface layer  38 , copper layer  48 , and solderability enhancement layer  18  may comprise a multitude of configurations. For instance, each layer may be formed in selected areas to improve solder ball bonding characteristics. More particularly, the interface layer  38  may only be deposited over the solder mask  20 , or over selected portions of the solder mask  20  to anchor the subsequent layers thereto. Similarly, the copper layer  48  and solderability enhancement layer  18  may be configured in different arrangements as well. Furthermore, aforementioned layers comprising the BPS solder ball pad structure may be disparate areas that are not contiguous or continuous. Thus, it may be desired to form separate copper regions that form the copper layer  48 . Likewise, separate solderability enhancement regions may, in combination, form the solderability enhancement layer  18 . Also, each layer is not required to be the same size as other layers. For instance, the solderability enhancement layer  18  may extend onto the vertical side of the copper layer  48 , or may extend laterally along the substrate surface beyond either the copper layer  48  and/or interface layer  38 . 
   Suitable and exemplary rigid insulative sheet  12  materials for a BPS substrate include BT832, MGC, MCL679, FR-4, FR-5 materials from Hitachi Co., Japan Suitable and exemplary flexible insulative sheet  12  material for a BPS substrate include polyimide layers or fibers such as UPILEX™ from Ube Industries Ltd., Japan, ESPANEX™ from Nippon Steel Chemical Co. Ltd., Japan, and KAPTON™ and MICROLUX™ commercially available from E.I. Dupont de Nemours Company, as well as Polytetrafluoroethylene (PTFE), and a liquid crystal polymer. It should also be noted that the term “sheet” as used herein encompasses not only a self-supporting structure but a layer of material supported on another structure. 
   AUS5, AUS308, AUS303, or AUS7 from Taiyo, Japan, and DSR2200 from Tamura, Japan, are examples of commercially available materials suitable for use in forming solder masks  20  for a rigid BPS substrate. AUS11, AUS21 and PSR8000FLX from Taiyo, Japan and CFP1122 and CFP1123 from Sumilite, Japan, are exemplary materials suitable for use with flexible BPS substrates. 
     FIG. 4A  shows a conventional SMD substrate  10  configuration having two solder ball mounting pads  28 , formed by apertures  19  defined by sidewalls  21  ( FIG. 4B ) of solder mask  20  that expose solder ball mounting pads  28  of the terminal pads  14  formed on the insulative sheet  12 , respectively. The distance between terminal pads  14  as well as tolerances in positioning the terminal pads  14  may substantially influence the amount of space in which to position conductive traces  30  and  32  extending between solder ball mounting pads  28 . The spacing between traces  30  and  32  is determined from a number of variables. The distance between the centers of the terminal pads  14 , termed “solder ball pad pitch,” the terminal pad diameter, the conductive trace thickness t, the number of conductive traces, the lateral tolerance in forming conductive traces  30  and  32  and terminal pads  14 , as well as the solder ball pad design all may influence the spacing d that may be afforded for placement of conductive traces  30  and  32  in relation to the terminal pads  14  on the insulative sheet  12 . In addition, it is common for the trace thickness t to be equal to the spacing between the traces. 
     FIG. 4B  shows a side cross-sectional view of the SMD substrate  10  shown in  FIG. 4A , but also including an example of a solder ball  24  (not shown in  FIG. 4A ) attached to the left-hand mounting pad  28 . The distance d between a terminal pad  14  and conductive trace  30 , conductive trace  30  and conductive trace  32 , as well as conductive trace  32  and another terminal pad  14  is shown. For ease of illustration, trace or line widths and space widths are taken to be substantially the same. Distance d, the spacing between a trace and another trace or a trace and a terminal pad for SMD-type solder ball pad configurations, may be determined by the following design rule: 
               d   SMD     =         S   pitch     -     P   dia     -     2   ⁢   Tol       N             Equation   ⁢           ⁢   1               
   Where: 
   d SMD  is the spacing between a trace and another trace or a trace and a terminal pad. 
   S pitch  is the solder ball pad pitch, or distance between the centers of the two pads. 
   P dia  is the terminal pad diameter. 
   Tol is the soldermask positional tolerance. 
   N is the number of spaces and traces required. 
   Applying Equation 1 to  FIGS. 4A and 4B , with assumed dimensions as follows, 
   S pitch =0.650 mm 
   P dia =0.300 mm 
   Tol=0.050 mm 
   N=5 (As can be seen in  FIG. 4B , the number of spaces “d” is 5 for two circuit traces and three intervening spaces between pads, assuming equal spacing and trace widths)
 
         d   SMD     =         .650   ⁢           ⁢   mm     -     .300   ⁢           ⁢   mm     -       2   ·   .050     ⁢           ⁢   mm       5         
 d SMD =0.050 mm  (two circuit traces)
 
   Comparatively,  FIGS. 5A and 5B  show a top and side cross-sectional view, respectively, of an embodiment of the BPS substrate  40  of the present invention. Although none of the drawings are drawn to scale and are for illustration purposes only, the sizes of the BPS mounting pads  36 , as defined by apertures  23 , respectively, are shown as substantially equal to the mounting pad  28  size as shown FIG.  4 A. However, the terminal pads  41  of the BPS substrate  40  are smaller than the terminal pads  14  of the SMD substrate  10  shown in  FIGS. 4A and 4B . Since the BPS mounting pads  36  are formed onto the top surface of the solder mask  20 , their outer extents do not influence the placement of conductive traces  30 ,  32  and  34 . Instead, the smaller terminal pads  41  may allow for additional spacing or additional conductive traces to be placed between BPS terminal pads  41 . Further, since the copper layer  48  extends over the solder mask  20 , it may be, for example, a diameter or width of at least 0.3 mm and preferably 0.35 mm to ensure good solder joint reliability using BPS mounting pads  36  without any reduction in circuit trace or spacing width. 
   Determining circuit trace spacing of the BPS solder ball pad configuration of the present invention may be accomplished by using Equation 1, used for SMD solder ball pad configurations; however, the terminal pad size may be reduced. 
   For instance, applying the design rule of Equation 1 to the embodiment of the present invention shown in  FIGS. 5A and 5B , with assumed dimensions as follows, 
   S pitch =0.650 mm 
   P dia =0.150 mm 
   Tol=0.050 mm 
   N=7 (As can be seen in  FIG. 5B , the number of spaces “d” is seven for three circuit traces and four intervening spaces between pads)
 
         d   BPS     =         .650   ⁢           ⁢   mm     -     .300   ⁢           ⁢   mm     -       2   ·   .050     ⁢           ⁢   mm       7         
 d BPS =0.057 mm
 
   Accordingly, the present invention may enable an increased number of traces to be placed between two solder ball pads of the present invention since the spacing size d may remain substantially identical to the spacing required for conventional bond pads having a smaller number of traces therebetween. Alternatively, additional space may be used to provide additional lateral clearance between the same number of traces; thus increased yield may result. 
   For example, employing the BPS solder ball configuration of the present invention wherein two traces extend between two solder ball pads, the spacing may be determined as follows: 
   S pitch =0.650 mm 
   P dia =0.150 mm 
   Tol=0.050 mm 
   N=5 (for two circuit traces and three intervening spaces)
 
         d   BPS     =         .650   ⁢           ⁢   mm     -     .150   ⁢           ⁢   mm     -       2   ·   .050     ⁢           ⁢   mm       5         
 d BPS =0.080 mm
 
   Thus, the present invention may enable more traces to be formed between solder ball pads of the present invention and/or alternatively, increased spacing between a trace and another trace or a trace and a terminal pad, as well as in circuit trace width, in relation to the conventional SMD pad configuration. 
     FIGS. 6A and 6B  show a conventional NSMD substrate  11  wherein two traces  30  and  32  extend between the terminal pads  14 .  FIG. 6A  shows apertures  19 ′ exposing the entire terminal pads  14 , the terminal pads  14  forming mounting pads  28 ′. 
     FIG. 6B  shows a side cross-sectional view of the solder ball pad configuration shown in  FIG. 6A , but also including an example of a solder ball  24  (not shown in  FIG. 6A ) attached to left-hand mounting pad  28 ′. The distance d between a terminal pad  14  and conductive trace  30 , conductive trace  30  and conductive trace  32 , as well as conductive trace  32  and a terminal pad  14  is shown. Distance d, the spacing between a trace and another trace or a trace and a terminal pad, is also commonly used as the trace width as well as the spacing distance between the vertical sidewall  21  of aperture  19 ′ and a terminal pad  14 . In addition, in an NSMD-type substrate, it is common for the design rule to specify a clearance distance between a vertical sidewall of an aperture, for instance, vertical sidewall  21  of aperture  19 ′, and the nearest sidewall of a trace, for instance, conductive trace  30 . 
   Distance d, for the NSMD solder ball pad configuration shown in  FIGS. 6A and 6B , may be determined by the following design rule: 
               d   NSMD     =         S   pitch     -     Aperture   dia     -     2   ⁢   Tol     -     2   ⁢   C       N             Equation   ⁢           ⁢   2             
 
   Where: 
   d NSMD  is the spacing between a trace and another trace or a trace and a terminal pad. 
   S pitch  is the solder ball pad pitch, or distance between the centers of the two pads. 
   Aperture dia  is the aperture diameter. 
   Tol is the solder mask positional tolerance. 
   C is the trace clearance, to ensure that the solder mask covers the trace. 
   N is the number of traces required and intervening spaces between the total number of traces. 
   For example, applying Equation 2 to two traces lying between two NSMD solder ball pads, as shown in  FIGS. 6A and 6B , the spacing may be determined as follows: 
   S pitch =0.650 mm 
   Aperture dia =0.300 mm 
   Tol=0.050 mm 
   C=0.030 mm 
   N=3 (for two traces and one intervening space between traces)
 
         d   NSMD     =         .650   ⁢           ⁢   mm     -     .300   ⁢           ⁢   mm     -       2   ·   .050     ⁢           ⁢   mm     -       2   ·   .030     ⁢           ⁢   mm       3         
 d NSMD =0.063 mm
 
   Referring back to  FIGS. 5A and 5B , the BPS substrate  40  of the present invention may offer increased spacing distance between elements formed on the surface of the substrate when compared to the SMD substrate  10  or the NSMD substrate  11  for an equal number of circuit traces. Also, since the BPS mounting pad  36  is positioned in part above the solder mask  20 , the BPS mounting pad size may offer a surface area equal to or greater than the SMD mounting pad  28  and/or the NSMD mounting pad  28 ′ sizes. 
   In addition, the BPS substrate  40  of the present invention may offer increased flexibility in design and improved bonding configurations. For instance, a BPS solder ball pad layer may be configured to further increase the surface area for attaching a solder ball thereto. More particularly, a BPS solder ball pad layer may be configured with scallops, radially extending fingers, apertures, or otherwise geometrically configured to increase the surface area or improve the bonding characteristics of a solder ball connection thereto. Further, a BPS solder ball pad layer may be configured to expose a portion of the terminal pad for connection to a solder ball in combination with the solder ball connection surface of the BPS solder ball pad layer. Additionally, as patterning of metal, and specifically copper, to define a solder ball pad layer is more accurate than solder mask patterning, a BPS solder ball pad layer offers superior positional and size tolerances as compared to an SMD solder ball mounting pad. 
     FIGS. 7A through 7L  show different embodiments  99 ,  101 ,  103 ,  105 ,  107  and  109  for a BPS solder ball pad layer  60  of the present invention wherein the older ball pad layer  60  comprises optional interface layer  38 , copper layer  48  as well as solderability enhancement layer  18 , but is shown as a single layer in  FIGS. 7A through 7L  for clarity.  FIGS. 7A and 7B  show embodiment  99  including a solder ball pad layer  60  configured generally in a circular area wherein the solder ball pad layer  60  includes an aperture  82  therethrough, exposing area  90  of terminal pad  41 . Area  90  may include a solderability enhancement layer  18 , although this is not shown for clarity. Thus, a solder ball attached to the BPS solder ball pad embodiment shown in  FIGS. 7A and 7B  may be affixed to the mounting pad  36 , mounting pad  36  comprising area  80  of solder ball pad layer  60  including side surface  7  and exposed area  90  of terminal pad  41 . 
     FIGS. 7C and 7D  show embodiment  101  including a solder ball pad layer  60  wherein solder ball pad layer  60  includes a terminal aperture  82  exposing area  90  of terminal pad  41 . In addition, scallops  73  are formed circumferentially about area  80  of solder ball pad layer  60 . Scallops, fingers, spokes, or other laterally extending shapes may be advantageous to increase the surface area of attachment for a solder ball, as well as provide additional vertical surfaces for attachment of a solder ball thereto. 
   For example  FIGS. 7E and 7F  show embodiment  103  including a solder ball pad layer  60  having a terminal aperture  82  exposing area  90  of terminal pad  41  and extending elements  81  configured as radially extending elements generally symetrically arranged about aperture  23 . 
   Also, as discussed hereinabove, since the centroid of the mounting surface of the solder ball pad influences the position of the solder ball, by employing the solder ball pad of the present invention, the solder ball pad layer  60  may be tailored to position solder balls as desired or to correct for inaccuracy in the placement of apertures in the solder mask  20 . More specifically, in the case where solder mask placement is less precise than solder ball pad layer  60  formation, the solder ball pad layer  60  may be used to correct variances in the solder ball mask aperture placement. Thus, each aperture in a solder mask  20  could be measured against a desired placement, and then the solder ball pad layer  60  could be displaced in order to correct for the deviation. Correction may occur prior to formation of the solder ball pad layer  60 ; thus, aperture  23  position may be determined prior to forming the solder ball pad layer  60  onto the substrate and the position of solder ball pad layer  60  corrected accordingly. Alternatively, the solder ball pad layer  60  of each mounting pad may be formed and then the solder ball pad layer  60  may be modified to position a solder ball in a desired position. For instance, laser ablation, selective etching, or other removal processes may be used to selectively modify the area of attachment of a solder ball pad, and thus adjust placement of a solder ball attached thereto. 
     FIGS. 7G and 7H  show embodiment  105  including a BPS solder ball pad configuration of the present invention where multiple apertures  77  are formed in solder ball pad layer  60 . Apertures  77  allow a solder ball to attach to the vertical sides thereof, thus increasing the surface area of attachment of a solder ball. Apertures  77  are shown as three circumferential slots that are positioned over the surface of solder mask  20 . 
     FIGS. 7I and 7J  show embodiment  107  including a BPS solder ball pad configuration of the present invention where individual regions  93 ,  95 ,  97 , and  111  of solder ball pad layer  60  as well as the exposed area  90  of the terminal pad  41  form the mounting pad  36 . Therefore, an attached solder ball will be affixed to areas  80  and  90  as well as side surface  71  and the side surfaces of regions  93 ,  95 ,  97 , and  111 . Such a configuration may be advantageous to provide more surface area for solder ball connection. 
   Many alternatives are possible, and the present invention is not limited to any one configuration. Individual solder ball pad layer  60  areas in combination with terminal pad  41  areas may form a mounting pad  36 . Although the present invention has been described herein as generally configured with a solder ball pad layer  60  that conforms to the solder mask, thus creating a vertical depression consistent with the aperture  23  in the solder mask  20 , the vertical surface of the solder ball mounting pad  36  may be tailored as desired. For instance, it may be advantageous to form the solder ball pad layer  60  so that it is substantially planar on its top surface. Conversely, it may be advantageous to create a vertical depression or tailor a vertical depression so as to create increased surface area or to promote bond strength or bond characteristics thereof. 
     FIGS. 7K and 7L  show embodiment  109  including a BPS solder ball pad configuration of the present invention where solder ball pad layer  60  forms areas  80  and includes aperture sections  113  that expose respective areas  90  of terminal pad  41 . Such a configuration may provide additional surface area and vertical sidewall attachment of a solder ball to areas  80  as well as provide an attachment area  80  to terminal pad  41 . Thus, the mounting pad  36  of the present invention may comprise areas of solder ball pad layer  60  in combination with areas of terminal pad  41 . 
   Referring now to  FIGS. 8A and 5B  of the drawings, a first exemplary process flow for fabricating the inventive structure of the present invention as depicted in  FIGS. 3A and 3B  is illustrated. As shown in  FIG. 8A , a polymer conductive adhesive forming optional interface layer  38  may be applied over solder mask  20  and into aperture  23 . By way of example only, suitable conductive polymers in the form of isotropic epoxy adhesives  3880  and  3889  are available from Henkel Loctite Corporation, Conn. The polymer interface layer  38  may be applied by stencil printing to cover the surface of terminal pad  41  exposed through aperture  23 , the sidewalls of solder mask  20  defining aperture  23  and the top horizontal surface of solder mask  20  to form a collar of the polymer around aperture  23 . An electroplated copper layer  48  may then be formed over interface layer  38 , followed by electroplating of a nickel layer  18   a  and a gold layer  18   b  together comprising solderability enhancement layer  18 , all as shown in FIG.  8 B. It is also contemplated that a metal interface layer  38  may be applied by stencil printing, followed by electroplating of the copper, nickel and gold layers. 
   Referring to  FIG. 9  of the drawings, an optional interface layer  38  to enhance adhesion to the solder mask  20  may be applied in the form of a nonconductive epoxy, such as the aforementioned HYSOL®EO1073 and EO1075 compounds or other suitable polymer, by stencil printing over the top surface of solder mask  20  surrounding aperture  23 , leaving the exposed area of terminal pad  41  and the sidewalls of solder mask  20  defining aperture  23  free of material. 
   In lieu of the use of a conductive or nonconductive polymer as an interface process, flow may proceed along several different paths. Referring to  FIG. 10A , a “coverlay” element in the form of a dry film photoresist  100  (positive or negative) may be applied over solder mask  20 , patterned by exposure to a required wavelength of light through a mask, developed, and portions of the dry film surrounding and extending into a aperture  23  removed to expose terminal pad  41  and define annulus  102  surrounding aperture  23 . If a nonconductive polymer has been used for adhesion enhancement to solder mask  20 , it may already be present on annulus  102 . As shown in  FIG. 10B , a copper seed layer  104 , which may comprise a metal interface layer  38 , may be electrolessly plated over the dry film photoresist  100  and into aperture  23 , covering the exposed portion of terminal pad  41 , which copper seed layer  104  may be augmented by electroplating, if desired. Dry film photoresist  100  is then stripped off mechanically or chemically as known in the art, removing with it the overlying copper and leaving the copper layer  48  within and surrounding aperture  23  contacting terminal pad  41  and extending from aperture  23  as a collar over the top surface of solder mask  20 , as shown in  FIG. 10C. A  nickel layer  18   a  and a gold layer  18   b  may then be electroplated onto the copper layer  48  to form solderability enhancement layer  18  and complete the structure of solder ball mounting pad  36  as shown in FIG.  10 D. 
   In another process sequence, and referring to  FIG. 11A , an optional nonconductive polymer interface layer  38  (not shown) may be applied to solder mask  20  in an area surrounding aperture  23 . In either case, with or without the presence of optional nonconductive interface layer  38 , an electroless copper seed layer  104 , which may itself comprise a conductive interface layer  38 , may be applied over solder mask  20 , into aperture  23  and over the exposed portion of terminal pad  41 . A coverlay element in the form of a dry film photoresist  100  (positive or negative) may be applied over solder mask  20 , patterned by exposure to a required wavelength of light through a mask, developed, and portions of the dry film surrounding and extending into aperture  23  removed to expose terminal pad  41  and define annulus  102  surrounding aperture  23  and exposing a portion of electroless copper seed layer  104 , as shown in FIG.  11 B. Copper may then be electroplated onto the exposed copper in annulus  102 , over sidewalls of solder mask  20  defining aperture  23  and onto the exposed portion of terminal pad  41  to complete copper layer  48 , as shown in FIG.  11 C. Nickel layer  18   a  and gold layer  18   b  may then be electroplated to form solderability enhancement layer  18 , again as shown in FIG.  11 C. Dry film photoresist  100  may then be stripped off mechanically or chemically, as known in the art, and the underlying electroless copper seed layer  104  removed by a soft etch comprising an alkaline ammonia solution to complete the fabrication of solder ball mounting pad  36 , as shown in FIG.  11 D. 
   Although the foregoing description contains many specifics, these should not be construed as limiting the scope of the present invention, but merely as providing illustrations of some exemplary embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. Features from different embodiments may be employed in combination. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the invention, as disclosed herein, which fall within the meaning and scope of the claims are to be embraced thereby.