Abstract:
A die-mounting substrate and method incorporating dummy traces for improving mounting film planarity makes the use of film attach possible with a simplified manufacturing process and in applications where film-attach was not previously practical. The die-mounting substrate includes dummy traces that are generated along with signal traces extending into the die mounting area of the substrate. The dummy traces are designed according to the same design rules as the signal traces and are disposed in otherwise empty regions between signal traces and vias within the die mounting area. The result is die mounting area without regions empty of signal traces that previously either lack conductor or are filled completely with conductor, either of which will result in surface variation that compromises the film bond.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuit packaging and more specifically, to a Method and substrate having improved die film-attach characteristics. 
     BACKGROUND OF THE INVENTION 
     Due to the increasing limitation of wafer-scale integration, in terms of cycle time and technology integration, significant advancements are being made in the area of multi-die packaging (e.g.: stacked-die package). Additionally, there is pressure from the semiconductor industry to reduce overall package size requirements that encompass both footprint area and package height. The package-height constraint results in thinner and thinner dies being stacked in a single package. However, there are limitations in the ability to stack dies of numerous sizes, shapes, and thickness in the desired configurations. 
     Currently, paste and elastomeric film adhesives are used to attach semiconductor die to the laminated substrate. The benefit of paste adhesives is that during the assembly process, the paste has viscous properties that accommodate irregular topographical features on the substrate and minimize the formation of die attach voids. 
     However, there are disadvantages to using paste adhesive. In particular, thin dies often have significant warpage. During the die attach process, the pick &amp; place tool will hold the die substantially flat while the die is in contact with the paste. However, due to the viscous nature of paste adhesives, when the pressure from the pick &amp; place tool is removed, the die will have a strong tendency to return to a warped shape. The result is undesirable non-uniform bond line thickness, which makes subsequent processing steps, such as wirebonding or die stacking, more difficult. Die attach paste also has a tendency to shrink during the curing process and the shrinkage may produce a reverse fillet that extends under the edge of the die rather than upward along the side walls of the die. 
     Film adhesives have the advantage of providing a uniform bond line thickness and do not shrink appreciably during the curing process. However, due to the rather rigid nature of film adhesives, topographical features on the substrate surface tend to cause interfacial voids between the bottom of the film adhesive and the surface of the substrate. The formation of such voids is undesirable, both from an assembly yield and reliability perspective. The presence of interfacial voids results in less than optimal adhesion and can trap moisture that expands and causes de-lamination during post-assembly temperature excursions. The voids are caused by the shapes of the surface variations themselves and by air that is trapped due to feature patterns that provide no exit for air during die mounting. 
     One method used to minimize topographical variations of the substrate is a fill of all areas around electrical traces with solid metal. However, solid regions of metal tend to be thicker that the metal traces due to etching/plating density, resulting in less than ideal planarity. Further, it is very important that the top solder mask layer and the underlying prepreg layer on either side of the top metal layer have sufficient contact to provide adhesion. 
     An alternative method of providing a substantially flat surface for attaching via film adhesive is planarizing the substrate, using a filler material and a mechanical grinding technique, prior to applying the top layer of solder mask. The disadvantage of mechanical planarization is a lack of technology available for high-volume manufacturing. Due to the need to incorporate filler material and due to the cost of the additional processing, it will likely be some time before the technique becomes a viable option and the industry fully adopts the technique. 
     Therefore, it would be desirable to provide a low cost and low processing overhead method and substrate for film-mounting semiconductor dies that overcomes problems associated with film-mounting a die over conductive pattern features on the surface of the substrate. 
     SUMMARY OF THE INVENTION 
     The above stated objectives are achieved in a method and substrate for film-mounting a semiconductor die. The substrate includes a plurality of dummy traces designed according to the same design rules as signal traces, with both the dummy traces and the signal traces disposed within a die mounting area. The dummy traces act as a fill pattern within regions empty of signal traces and vias, and any other functional conductive pattern features disposed within the die mounting area. 
     The presence of the dummy traces improves the planarity of the die mounting area of the substrate by filling the otherwise empty regions with a pattern consistent with the signal trace pattern, thus overcoming irregularities of either empty regions or regions completely filled with metal, either of which will adversely affect the planarity of the substrate in the die mounting area. The dummy traces are also disposed so that channels between the dummy traces and/or signal traces provide a path for air to exit from under the die during tape mounting. 
     A solder mask may be applied to the substrate over the signal traces and dummy traces, with the surface variations of the signal traces and dummy traces producing a corresponding variation of the outer surface of the solder mask so that channels are provided for the exit of air from under the die. 
     The substrate is then incorporated in a semiconductor package that includes an adhesive film that attaches a die to the substrate over the die mounting area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a substrate according to an embodiment of the present invention. 
         FIG. 2  is an enlarged plan view of region  2  of  FIG. 1 . 
         FIG. 3  is an enlarged plan view of region  3  in  FIG. 1 . 
         FIG. 4  is a sectional view of the substrate of  FIG. 1  as taken across line  4 - 4 . 
         FIG. 5  is an enlarged plan view of region  5  in  FIG. 1 . 
         FIG. 6  is an enlarged plan view of region  6  in  FIG. 1 . 
         FIG. 7  is a longitudinal sectional view of a signal trace in a substrate according to an embodiment of the present invention. 
         FIG. 8  is a transverse cross-section view of signal traces and dummy traces according to an embodiment of the present invention. 
         FIG. 9  is a plan view illustrating a semiconductor package according to an embodiment of the present invention. 
         FIG. 10  is a cross section view of a semiconductor package according to an embodiment of the present invention. 
         FIG. 11  is a cross section view of another semiconductor package according to another embodiment of the present invention. 
         FIG. 12  is a cross section view illustrating attachment of a semiconductor die to a substrate by a method according to an embodiment of the present invention. 
         FIG. 13  is a cross section view illustrating wire-bonding between a semiconductor die and a substrate according to an embodiment of the present invention. 
         FIG. 14  is a cross section view illustrating encapsulation of an upper portion of a substrate according to an embodiment of the present invention. 
         FIG. 15  is a cross section view illustrating fusion of solder balls to lower portions of a substrate according to an embodiment of the present invention. 
     
    
    
     The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout. 
     DETAILED DESCRIPTION 
       FIG. 1  is a plan view of a substrate  100  according to an embodiment of the present invention. Substrate  100  includes an insulating layer  110 , a plurality of signal traces  120 , and a plurality of dummy traces  130 , formed on insulating layer  110 . Signal traces  120  and dummy traces  130  have prescribed width, spacing and length, according to design rules. Surfaces of signal traces  120 , dummy traces  130 , and a non-trace portion of insulating layer  110  are coated with a solder mask (not shown). Insulating layer  110  is shown as a substantially rectangular thin planar plate, having four sides and four corners and may be square in shape. Insulating layer  110  may be formed of typical insulating film, insulating tape, thermosetting resin, prepreg material, or their equivalents, and the material for insulating layer  110  is not limited with respect to the present invention. 
     Each of signal traces  120  has a bond finger  121  formed at a portion thereof which is located at a region nearer to the side edge of insulating layer  110 . Bond finger  121  has a width larger than the other portion of signal trace  120  (excepting any vias) and serves as the portion to which a die-connect wire is bonded in the manufacturing process of the semiconductor package. The surface of the bond finger  121  may be plated with gold, silver, palladium, nickel, or an equivalent, in order to enhance the adhesion strength and reliability of the bond between the bond finger and the wire. Bond finger  121  is not coated with solder mask, so that solder mask does not have to be removed from bond finger  121  during or prior to bonding. Signal trace  120  may include a circular via pad  122 , formed at a distance apart from bond finger  121 , and which is wider than other portions of signal trace  120 . 
     Dummy traces  130  are formed in the otherwise empty regions between signal traces  120 . Dummy traces  130  define channels, which enable air to travel from the central portion of insulating layer  110  to the edge thereof, without being trapped during the adhesion process of the semiconductor die. The channels are normally present between signal trace  120  and adjacent signal traces  120 , and in the present invention, the channel spacing and width are maintained across the die mounting area by dummy traces  130 , and extend toward the side edges or the corners of insulating layer  110 . Additionally, dummy traces  130 , by filling open areas in the design, yield a more uniform mounting surface that promotes adhesion and minimizes the formation of interfacial voids during the adhesion process. 
     Dummy traces  130 , as described above, have the same width and spacing as those of signal traces  120 , so that dummy traces  130  can be formed by the same design rules for signal traces  120 . For example, when signal trace  120  has a width of 40˜80 um, dummy trace  130  also has a width of 40˜80 um. Further, the spacing between signal trace  120  and a dummy trace  130  adjacent to signal trace  120 , and the spacing between dummy traces  130  is equal to the minimum spacing between signal trace  120  and another signal trace  120 . For example, if the minimum spacing between signal trace  120  and an adjacent signal trace  120  is 40 um, the spacing between dummy trace  130  and the next dummy trace  130  should also be 40 um. Further, a solid region  135  having a predetermined area may be formed between two adjacent signal traces  120 , between signal trace  120  and dummy trace  130 , and/or between two adjacent dummy traces  130 . However, it is preferred that solid region  135  has an area as small as possible. For example, solid regions may be formed by combining dummy traces that have lengths that are less than 5 times the minimum width of signal traces  120 . Traces having widths less than 30 um and greater than 80 um are also contemplated by the present invention. However, at the time of this application, normal trace widths range from 40 um to 80 um. 
     Reference numeral  111  denotes a line along which punch singulation or saw singulation is performed during the manufacture of a semiconductor package that includes substrate  100 . 
     Several selected regions of substrate  100 , detailing specific features of the present invention, will be described below. However, the particular regions selected and their associated description are not limitations of the present invention, but are intended to be exemplary only. The present invention is directed in general toward an arrangement of signal traces  120  and dummy traces  130  for minimizing formation of non-patterned regions between signal traces  120 , and may be implemented in various forms. The arrangement of traces in accordance with embodiments of the present invention serves to minimize any topographical features on the substrate surface that will tend to cause interfacial voids between the bottom of a film adhesive that is used to attach a die, and the surface of the substrate. 
       FIG. 2  shows details of region  2  of  FIG. 1 . Specifically,  FIG. 2  is a plan view showing an illustrative signal trace  120  of substrate  100  according to an embodiment of the present invention. As shown, signal trace  120  has a bond finger  121  formed at one end and a via pad  122  formed at the other end of signal trace  120 . Bond finger  121  is wider than the connecting portion of signal trace  120  and via pad  122  is wider than the bond finger  121 . Further, signal trace  120  may be bent at least once at an obtuse angle, i.e., an angle between 90° and 180°. 
       FIG. 3  shows region  3  of  FIG. 1 . Specifically,  FIG. 3  is a plan view showing illustrative dummy traces  130  formed adjacent to signal trace  120  of substrate  100 , according to an embodiment of the present invention. As shown, a plurality of dummy traces  130  are formed adjacent to signal trace  120 , i.e., in the otherwise region between and defined by the extension of signal traces  120 , including any vias. Each of dummy traces  130  has a shape similar to that of signal trace  120 . For example, each of dummy traces  130  may be bent at least once in parallel to signal trace  120 , so that dummy traces  130  follow a curvilinear or segmented linear path along and between signal traces  120 . Further, signal trace  120  and dummy trace  130  have substantially the same width. For example, a same width between 40 and 80 um. Further, the spacing between adjacent traces (dummy traces  130  and/or signal traces  120 ) may be substantially the same. For example, all of the inter-conductor spacings may be a same value between 40 and 80 um. However the use of smaller lines and spaces (&lt;40 um) are also contemplated and using these smaller dimensions generally results in a more planar substrate surface and therefore more desirable properties for film-adhesion. However, in certain instances, cost and/or yield may prohibit the use of fine lines and spaces. 
     By incorporating dummy traces  130 , the present invention provides for an interface between the film adhesive on the backside on the semiconductor die and the substrate surface that is substantially void free. Further, an improved adhesion force between the film adhesive on the backside of the semiconductor die and the substrate is established while using the current design and processing techniques of substrate  100 . 
     As shown in  FIG. 4 , a solid region  135  may be formed to fill small open regions. For example, when a small group of dummy traces  130  have lengths less than 5 times the minimum width of signal traces  120 , these dummy traces may be combined to form a solid region of metal. This will reduce the possibility of dummy traces lifting during substrate processing operations. Solid region  135  has a width larger than the width of dummy trace  130 . However, when solid region  135  has too large an area, the adhesion between the solder mask  140  and insulating layer  110  may be reduced to an unacceptable level. Therefore, formation of solid regions  135  is restricted to particular conditions, an example of which is provided in the above description. In the drawings, reference numeral  141  is a solder mask coated at the bottom surface of insulating layer  110 . 
       FIG. 5  shows details of region  5  in  FIG. 1 . Specifically,  FIG. 5  is a plan view showing other illustrative dummy traces  130  extending perpendicularly to the side edge  112  of insulating layer of the substrate  100  according to an embodiment of the present invention. 
     As shown, dummy traces  130  may be formed to extend perpendicularly to the side edge  112  of insulating layer. Therefore, channels formed between dummy traces  130  also extend perpendicularly to the side edge  112  of insulating layer, allowing air that would be trapped by a more closed structure to escape toward the edge of insulating layer  110 . 
       FIG. 6  shows details of region  6  in  FIG. 1 . Specifically,  FIG. 6  is a plan view showing other illustrative dummy traces  130  extending at an angle with respect to the side edge  112  of insulating layer of substrate  100 , in accordance with an embodiment of the present invention. 
     As shown, dummy traces  130  may be formed to extend at an angle with respect to the side edge  112  of insulating layer. Therefore, channels formed between dummy traces  130  also extend at an angle with respect to the side edge  112  of insulating layer, thereby allowing air, which may be otherwise trapped in a central portion thereof, to easily escape toward the edge of insulating layer  110 . 
     It should be appreciated that signal traces  120  and dummy traces  130  patterned as described in the detail drawings above are only representative selected examples and may be modified into various types and shapes within the scope of the present invention. The main purpose of the dummy trace fill design in conjunction with film die attach adhesive is to provide a uniform, substantially flat surface to minimize the formation of interfacial voids that could otherwise occur due to trapped air, and to provide channels for air to escape from under the die at the die edge. 
       FIG. 7  is a longitudinal cross section view of a signal trace  120  in substrate  100  according to an embodiment of the present invention. As shown, signal trace  120  is formed with a predetermined thickness on the surface of insulating layer  110  and has a bond finger  121  formed at one end thereof and a via pad  122  formed at the other end thereof. Further, a solder mask  140  is applied with a predetermined thickness covering signal trace  120 . Bond finger  121  of signal trace  120  is exposed to the upper exterior through the solder mask  140 . Therefore, a conductive wire may be bonded to the bond finger  121 . A ball land  124  may also be formed on the bottom surface of insulating layer  110 . Ball land  124  is electrically connected to via pad  122  of signal trace  120  through conductive via  123 . Conductive via  123  penetrates insulating layer  110  to provide electrical connection between signal trace  120  and ball land  124 . A solder mask  141  is applied with a predetermined thickness on the bottom surface of insulating layer  110  and ball land  124  of insulating layer  110  is exposed to the lower exterior through solder mask  141 . Thereafter, a solder ball may be reflowed to the ball land  124 . 
     Although a multi-layer structure is not shown in the drawings, the present invention can be applied to a circuit substrate having one or more conductive layers. In such a circuit, dummy traces  130  will occupy the layer(s) of the substrate to which a semiconductor die is attached. 
       FIG. 8  is a transverse sectional view of signal traces  120  and dummy traces  130  in the substrate  100  according to an embodiment of the present invention. 
     As shown, signal traces  120  and dummy traces  130 , having a predetermined width and a predetermined spacing, are formed on the surface of insulating layer  110 . A solder mask  140  is applied with predetermined thickness to cover the surfaces of insulating layer  110 , signal traces  120  and dummy traces  130 . The surface of the solder mask  140  has a surface, which is lowest on the surface of insulating layer  110  and highest on the surface of signal traces  120  and dummy traces  130 , as the presence of conductive metal features atop insulating layer  110  cause variations in the height of solder mask  140 . Therefore, the solder mask  140  also defines channels at its top surface, located between the portions of solder mask  140  that cover the traces. In particular, when attaching a semiconductor die, channels in the solder mask may provide for the escape of air to edges of the die. Since the channels formed in solder mask  140  follow a pattern according to the shapes and construction of the channels formed between both signal traces  120  and dummy traces  130 , the air can escape without being trapped during the adhesion step for the semiconductor die just as in the examples described above without solder mask  140 . Since solder mask  140  has a nearly flat surface overall, the solder mask provides a very uniform and stable mounting surface atop which stacking of a semiconductor die can be performed. Also, it is preferred that the area at which the solder mask  140  and insulating layer  110  are in direct contact with each other is between 40% and 60% of the entire area for which the solder mask  140  and insulating layer  110  are separated by conductive features (e.g, signal traces  120  and dummy traces  13 ). Generally, it is preferred that the overall area of signal traces  120  (including vias), dummy traces  130  and solid regions  135  formed on insulating layer  110  occupies approximately 50% of the entire area of insulating layer  110 . According to the above conditions, a sufficient adhesive force between the solder mask and insulating layer  110  can be assured. 
       FIG. 9  is a plan view illustrating a semiconductor die placement (semiconductor die mounting area) on a substrate according to an embodiment of the present invention. Reference numeral  150  denotes an outline of a semiconductor die, which is attached to substrate  100  by an adhesive film (not shown) that is applied to the lower surface of the semiconductor die. Semiconductor die has a size small enough to allow semiconductor die to fit within the plurality of bond fingers  121  formed on substrate  100 , so that bond fingers  121  are exposed after die mounting. Signal traces  120  and dummy traces  130  have the same width and the same spacing between them, as described above. Further, as described above, the channels formed between the traces are oriented to extend substantially toward the edge of the substrate  100 , so that the air between the adhesive film and the solder mask can easily escape during attachment of the semiconductor die  150 . Thus, the formation of interfacial voids between the adhesive film and the solder mask is minimized to prevent de-lamination of the die from substrate  100  during subsequent thermal processing steps. 
       FIG. 10  is a cross section view of a semiconductor package according to an embodiment of the present invention. As shown, a semiconductor package  101  according to an embodiment of the present invention includes a substrate  100 , a semiconductor die  150 , an adhesive film  160 , a conductive wire  170 , an encapsulant  180 , and a solder ball  190 . Substrate  100  includes at least one signal trace  120  and at least one dummy trace  130  formed on the surface of insulating layer  110 , but in practice the number of signal traces  120  and dummy traces will be on the order of 100 or more. Signal trace includes bond finger  121  and a via pad  122 . Dummy traces  130  are formed between signal traces  120 . Signal traces  120  and dummy traces  130  may be formed to have the same width and the same spacing between them. Specifically, signal traces  120  and dummy traces  130  may have a width having a value between 40 and 80 um. Further, both the spacing between a signal trace  120  and a dummy trace  130  most adjacent to signal trace  120  and the spacing between a dummy trace  130  and another dummy trace  130  most adjacent to dummy trace  130  may be a same value between 40 and 80 um. Bond finger  121  of signal trace  120  is exposed to the upper exterior of solder mask  140 . Via pad  122  of signal trace  120  may be connected to ball land  124  of the bottom surface of insulating layer  110  through conductive via  123 . Ball land  124  is not coated with solder mask  141  so that a solder ball can be later re-flow soldered onto ball land  124 . Dummy trace  130  may also include a via pad (not shown). Solder mask  140  formed over signal trace  120  and dummy trace  130  has a wave-like sectional shape in detailed view due to the thickness of signal trace  120  and dummy trace  130 . However, solder mask  140  has a substantially flat surface overall. 
     Semiconductor die  150  with a plurality of bond pads  154  formed on the upper surface thereof is located on the substrate  100 . Semiconductor die  150  is located on solder mask  140  where solder mask covers the plurality of signal traces  120  and the plurality of dummy traces  130 . An adhesive film  160  is located between the lower surface of the semiconductor die  150  and the substantially flat solder mask  140 , so as to attach and fix the semiconductor die  150  to substrate  100 . In microscopic view, a plurality of channels are formed in the surface of solder mask  140  above signal traces  120  and dummy traces  130 , so as to allow for the escape of air, thereby inhibiting formation of interfacial voids between adhesive film  160  and solder mask  140 . 
     After die mounting, bond pads  154  of semiconductor die  150  are electrically connected through by conductive wire  170  to the bond fingers  121  of signal traces  120  that are exposed through solder mask  140 . Conductive wire  170  may be formed of usual gold wire, aluminum wire or their equivalents, but the materials of conductive wire  170  are not limitations of the present invention. Next, semiconductor die  150  and conductive wire  170  are encapsulated by an encapsulant  180 , to protect them from the external environment. Finally, solder balls  190  are fused to ball lands  124  exposed through the bottom surface of insulating layer  110  of the substrate  100 . Solder balls  190  serve as terminals that enable mounting of substrate  100  on an external device or circuit board. 
     In the semiconductor package described above, semiconductor die  150  is attached onto a substantially flat substrate  100  having a plurality of signal traces  120  and a plurality of dummy traces  130  by an adhesive film  160 . The bonding line of the semiconductor die  150  can thereby be uniformly maintained. Moreover, since nearly no void is formed between the adhesive film  160  and the solder mask  140 , adhesive film  160  has a superior adhesion strength in the application. 
       FIG. 11  is a cross section view of a semiconductor package  102  according to another embodiment of the present invention. In semiconductor package  102 , a first semiconductor die  151  is attached to substrate  100  according to the above descriptions. Since the mounting of semiconductor die  151  is very stable and co-planar with insulating layer  100 , another semiconductor die  152  can easily be stacked onto first semiconductor die  151 . Semiconductor die  152  is attached to the lower semiconductor die  151  by means of adhesive paste or an adhesive film  160 . At least one additional semiconductor die  152  can be stacked on first semiconductor die  151 , because first semiconductor die  151  provides a flat and stable mounting surface. 
       FIG. 12  is a cross sectional view illustrating attachment of a semiconductor die to a substrate according to an embodiment of the present invention. As shown, the semiconductor die  150  with an adhesive film  160  applied, on the lower surface of the semiconductor die  150  is attached to the upper surface of the substrate  100 . Substrate  100  has signal traces  120  and dummy traces  130  having a predetermined width and a predetermined spacing between them but has a substantially flat surface. Specifically, all of signal traces  120  and dummy traces  130  are formed to have the same width and the same spacing between them and the channels formed between them are preferably oriented toward the edge from the center, so that the air can escape out from under semiconductor die  150  through the channels during attachment of semiconductor die  150  to substrate  100 . Therefore, after the adhesion of the semiconductor die  150 , a minimal number/size of voids are formed between adhesive film  160  and substrate  100 . Also, the bonding line of the semiconductor die  150  is very uniformly maintained. In other words, the semiconductor die  150  is attached in a highly flat state while securing a wide adhesion area. 
       FIG. 13  is a sectional view illustrating wire-bonding between a semiconductor die  150  and a substrate  100  according to an embodiment of the present invention. As shown, a bonding pad  154  formed at the periphery of semiconductor die  150  is connected through a wire  170  to a bond finger  121  formed at signal trace  120  of the substrate. Wire  170  may be formed of usual gold wire, aluminum wire or their equivalents, and the material of conductive wire  170  is not a limitation of the present invention. 
       FIG. 14  is a sectional view illustrating encapsulation of an upper portion of a substrate according to an embodiment of the present invention. As shown, both semiconductor die  150  and conductive wire  170  on substrate  100  are encapsulated by an encapsulant  180 , so that semiconductor die  150  and conductive wire  170  are protected from the external environment. 
       FIG. 15  is a sectional view illustrating fusion of solder balls to lower portions of a substrate according to an embodiment of the present invention. As shown, solder balls  190  are fused to a ball land  124  exposed through insulating layer  110  of the substrate  100 . Semiconductor package  100  is manufactured through the above-described steps and is then mounted to an external device. Further, a minimal size/number of voids are formed between adhesive film  160  and substrate  100 , thereby minimizing the chance of de-lamination caused by heat generated in the step of mounting the semiconductor package  100  to an external device or heat generated by the operation of semiconductor die  150 . 
     It is contemplated that the present invention can be applied to circuit substrates having more than one conductive layer. In such a circuit, dummy traces  130  will occupy the layer(s) of the substrate to which at least one semiconductor die is attached. Additionally, solder balls are connected to the conductive layers by means of conductive vias provided through insulating layer(s). 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process may be implemented by one of skill in the art in view of this disclosure.