Abstract:
Electroless nickel bumps of die pads and a method thereof are disclosed. A protection layer is formed on the top surface and a surrounding sidewall of each electroless nickel bump in turn or at the same time by two separated processes or the same process. The two separated processes are selected from the group consisting of immersion gold and immersion silver. Thereby hardness of the top surface of the electroless nickel bump is improved and reduced. Moreover, easy oxidation of the surrounding sidewall of the electroless nickel bump and short circuit of the bump caused by electron migration can both be avoided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to electroless nickel bumps of die pads and a manufacturing method thereof, especially to electroless nickel bumps of die pads and a manufacturing method thereof in which an outer protection layer is formed separatedly or simultaneously on a top surface and a surrounding sidewall of each electroless nickel bump by immersion gold or immersion silver process so as to improve and reduce hardness of the electroless nickel bump. Moreover, easy oxidation of the surrounding sidewall of the electroless nickel bump and short circuit of the bump caused by electron migration can both be avoided. Thus the manufacturing processes are simplified, the production cost is reduced and the quality is stable. 
         [0002]    In the fields of link (such as bumps), package or manufacturing process of semiconductor chip or wafer, there are various techniques available now such as TW M397591, M352128, M412460, M412576, M410659, I306638, I320588, I255538, I459362, I253733, I273651, I288447, I295498, I241658, I259572, I472371, I24286, I269461, I329917, I282132, I328266, I284949, and the U.S. Pat. No. 8,030,767, U.S. Pat. No. 7,981,725, U.S. Pat. No. 7,969,003, U.S. Pat. No. 7,960,214, U.S. Pat. No. 7,847,414, U.S. Pat. No. 7,749,806, U.S. Pat. No. 7,651,886, U.S. Pat. No. 7,538,020, U.S. Pat. No. 7,750,467, U.S. Pat. No. 7,364,944, U.S. Pat. No. 7,019,406, U.S. Pat. No. 6,507,120, U.S. Pat. No. 7,999,387, U.S. Pat. No. 7,993,967, U.S. Pat. No. 7,868,470, U.S. Pat. No. 7,868,449, U.S. Pat. No. 7,972,902, U.S. Pat. No. 7,960,825, U.S. Pat. No. 7,952,187, U.S. Pat. No. 7,944,043, U.S. Pat. No. 7,934,313, U.S. Pat. No. 7,906,855, etc. These prior arts all have slight improvement. Thus compared with the crowded arts, the present invention has an inventive step even with just a little progress. 
         [0003]    In conventional processes, a metal layer is formed on die pads by Under Bump Metallization (UBM) before formation of the bumps. Then the bumps are formed on the metal layer of the die pads by metal electroplating or printing of silver paste. Both the production cost and the manufacturing difficulty are higher. Moreover, the manufacturing processes are complicated and the yield rate is reduced. The formation of the bumps also needs more precious metal materials. 
         [0004]    Furthermore, the bump made from silver paste has larger range of hardness. That means the hardness can be adjusted by heating conditions. However, the conventional electroless nickel bump has smaller hardness range. That means the surface hardness of the electroless nickel bump is larger and unable to be adjusted by heating conditions. This has negative effect on the following adhesion process. 
         [0005]    In addition, the electroless nickel bump is easy to get oxidized without being covered by a sidewall protection layer. At the same time, short circuit occurs between the bumps due to electron migration. Although a part of prior arts such as TW M410659, US2011/0260300591, and TW M397591, etc., have revealed the structure or method for manufacturing the sidewall protection layer, these structure and methods are complicated. The production cost is unable to be reduced without simplifying the manufacturing processes. This also has negative effect on mass production. Generally, the electroless nickel bumps are conventionally produced without the photoresist covering. Although the electroless nickel bumps produced have the sidewall protection layers, the height of the electroless nickel bump is smaller (such as 2-10 μm) and the distance between two adjacent bumps is limited. Thus the bumps are unable to be produced delicately and the practical requirements of the product in the field are unable to be met. 
         [0006]    There is room for improvement and a need to provide bumps with sidewall protection layers, produced by simple processes and reduced cost. The surface hardness of the bumps meets requirements of following adhesion process. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore it is a primary object of the present invention to provide electroless nickel bumps of die pads and a method thereof. Under the condition with photoresist coating, the bumps  30  with a certain height are formed on the surface of a catalyst layers on surface of the die pads respectively by electroless nickel plating. The bump is made from electroless nickel. Then use two separated manufacturing processes or the same manufacturing process selected from immersion gold and immersion silver process to form outer protection layers on a top surface and a surrounding sidewall of the bumps respectively or simultaneously so as to cover the exposed surface of the electroless nickel bumps completely. The outer protection layer includes at least one protection layer made from one of the materials selected from immersion gold or electroless silver. Thereby the hardness of the electroless nickel bump is improved and reduced. Moreover, the easily-oxidized problem of the surrounding sidewall of the electroless nickel bump and the short circuit problem caused by electron migration can also be solved. The manufacturing processes are simplified, the production cost is reduced and the quality of the product is stable. 
         [0008]    In order to achieve the above object, electroless nickel bumps of die pads according to the present invention includes a die, a plurality of catalyst layers, a plurality of electroless nickel bumps, and a plurality of protection layers. The die consists of a surface, a plurality of die pads disposed on the surface, and a protection layer that is formed on the surface and having a plurality of openings. The die pad is exposed through the opening correspondingly. The catalyst layers are formed on the surface of the die pads respectively by under bump metallization (UBM) or zincating. The bumps with a certain height and made from electroless nickel are formed on the surface of the catalyst layers of the die pads by electroless nickel plating under the condition with photoresist coating. The protection layers are formed on a top surface and a surrounding sidewall of the bumps respectively or simultaneously by two separated manufacturing processes or the same manufacturing process selected from immersion gold and immersion silver process so as to cover the surface of the electroless nickel bumps completely. The protection layer includes at least one protection layer made from immersion gold (IG) or electroless silver (ES). Thereby the hardness of the electroless nickel bump is reduced. Moreover, oxidation of the surrounding sidewall of the electroless nickel bump and short circuit caused by electron migration can both be avoided. Therefore the manufacturing processes are simplified, the production cost is reduced and the quality of the product is stable. 
         [0009]    A manufacturing method of electroless nickel bumps of die pads includes a plurality of steps as follows. 
         [0010]    First, provide a die with a surface while a plurality of die pads is disposed on the surface and a first protection layer with a plurality of openings for exposure of the die pads correspondingly is formed on the surface. 
         [0011]    Then form a photoresist layer on the first protection layer and pattern the photoresist layer so as to have a plurality of openings corresponding to each die pad and a part of the first protection layer surrounding the die pad. 
         [0012]    Next form a catalyst layer on a surface of each die pad by a process selected from the group consisting of under bump metallization (UBM) and zincating. 
         [0013]    Form an electroless nickel bump in each of the openings by electroless nickel plating. 
         [0014]    Form a top surface protection layer on a top surface of each electroless nickel bump by a process selected from the group consisting of immersion gold and immersion silver under the condition with the photoresist layer. The top surface protection layer includes at least one protection layer made from a material selected from the group consisting of immersion gold (IG) and electroless silver (ES). 
         [0015]    Remove the photoresist layer to expose the top surface protection layers, the electroless nickel bumps (the surrounding sidewall thereof) and a part of the first protection layers not beyond the bumps. 
         [0016]    At last, form a sidewall protection layer on a surrounding sidewall of each electroless nickel bump by a process selected from the group of immersion gold and immersion silver. The sidewall surface protection layer includes at least one protection layer made from a material selected from the group consisting of immersion gold (IG) and electroless silver (ES). 
         [0017]    In the above method, the plurality of top surface protection layers and sidewall protection layers are formed by two separated processes. The top surface protection layers are first formed on top surface of the electroless nickel bumps by a process selecting from the group consisting of immersion gold and immersion silver. Then a sidewall protection layer is formed on a surrounding sidewall of each electroless nickel bump by a process selecting from the group consisting of immersion gold and immersion silver. 
         [0018]    Furthermore, another embodiment of a manufacturing method of electroless nickel bumps of die pads according to the present invention includes a plurality of steps as follows. 
         [0019]    First, provide a die with a surface while a plurality of die pads is disposed on the surface and a first protection layer with a plurality of openings for exposure of the die pads correspondingly is formed on the surface. 
         [0020]    Then form a photoresist layer on the first protection layer and pattern the photoresist layer so as to make the patterned photoresist layer have a plurality of openings corresponding to each die pad and a part of the first protection layer surrounding the die pad. 
         [0021]    Next form a catalyst layer on a surface of each die pad by a process selected from the group consisting of under bump metallization (UBM) and zincating. 
         [0022]    Form an electroless nickel bump in each of the openings by electroless nickel plating. 
         [0023]    Remove the photoresist layer to expose the electroless nickel bumps, and a part of the first protection layers not beyond the bumps. 
         [0024]    At last, form an outer protection layer on a top surface and a surrounding sidewall of each of the electroless nickel bumps respectively at the same time by a process selected from the group consisting of immersion gold and immersion silver while the outer protection layer includes at least one protection layer made from a material selected from the group consisting of immersion gold (IG) and electroless silver (ES). 
         [0025]    In the method mentioned above, the outer protection layer is formed on the top surface of and the surrounding sidewall of each electroless nickel bump simultaneously by the same process selected from the group consisting of immersion gold and immersion silver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic drawing showing a cross section of an embodiment of electroless nickel bumps of die pads according to the present invention; 
           [0027]      FIG. 2  is a schematic drawing showing a cross section of an embodiment of electroless nickel bumps of die pads according to the present invention; 
           [0028]      FIG. 3  is a schematic drawing showing a cross section of an embodiment of electroless nickel bumps of die pads according to the present invention; 
           [0029]      FIG. 4  is a schematic drawing showing a cross section of an embodiment of electroless nickel bumps of die pads according to the present invention; 
           [0030]      FIG. 5A  to  FIG. 5G  are schematic drawings showing cross sections of an embodiment of electroless nickel bumps of die pads formed by two separated processes during manufacturing processes according to the present invention; 
           [0031]      FIG. 6A  to  FIG. 6F  are schematic drawings showing cross sections of an embodiment of electroless nickel bumps of die pads formed by the same process at the same time during manufacturing processes according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Refer from  FIG. 1  to  FIG. 4 , electroless nickel bumps of die pads  1  according to the present invention includes a die  10 , a plurality of catalyst layers  20 , a plurality of electroless nickel bumps  30 , a plurality of top surface protection layers  40  and a plurality of sidewall protection layers  60 . 
         [0033]    The die  10  consists of a surface  11 , a plurality of die pads  12  disposed on the surface  11  and a first protection layer  13  formed on the surface  11  and having a plurality of openings  14 . The die pad  12  is exposed through the opening  14  correspondingly. The die  10  is generally produced by wafer fabrication factories. The die pad  12  layout on the surface is not limited. The layout can be designed into various arrays according to client&#39;s requirements. The first protection layer  13  is made from nitride. 
         [0034]    The catalyst layers  20  are formed on the surface of the die pads respectively by under bump metallization (UBM) or zincating. The catalyst layer  20  is mainly used for connecting the die pads  12  and worked as deposition medium for following electroless metal deposition so as to form the electroless nickel bumps  30 . In an embodiment of the present invention, the catalyst layer  20  made from zinc is produced by using 15-30% (w/w) aqueous solutions of zinc salts at 20-35 degrees Celsius for 10 seconds to 60 seconds. 
         [0035]    The bumps  30  with a certain height are formed on the surface of the catalyst layers  20  on surface of the die pads  12  respectively by electroless nickel plating. Thus the bump  30  is called electroless nickel bump  30 . In this embodiment, the thickness of the bump  30  is formed due to deposition of electroless nickel (electroless deposition of nickel) with photoresist coating. Thus the height of the bump  30  meets the design requirement. The nickel salts are used and nickel (II) phosphate is preferred. This is due to that nickel (II) phosphate has self-catalytic property so as to increase the thickness (height) of the electroless nickel bump  30 . In this embodiment, the bump  30  made from electroless nickel is produced by using 4-6.5 g/L aqueous solutions of nickel salts at 75-100° C. for 30 minutes to 75 minutes. 
         [0036]    The top surface protection layers  40  are formed on the top surfaces of the electroless nickel bumps  30  respectively. The top surface protection layer  40  includes at least one protection layer made from materials selected from the group consisting of immersion gold (IG) and electroless silver (ES). The top surface protection layer  40  is formed on top surface of each electroless bump  30  by immersion gold process or immersion silver process under the condition with or without a patterned photoresist layer  50 . The sidewall protection layers  60  are arranged at surrounding sidewalls of the electroless nickel bumps  30  respectively. The sidewall protection layer  60  includes at least one protection layer made from materials selected from the group consisting of immersion gold (IG) and electroless silver (ES). The sidewall protection layer  60  is formed on the surrounding sidewall of each electroless bump  30  by immersion gold process or immersion silver process under the condition without a patterned photoresist layer  50 . Both the top surface protection layer  40  on the top surfaces of each electroless nickel bump  30  and the sidewall protection layer  60  on the surrounding sidewall of each electroless nickel bump  30  are covered over the exposed surface of each electroless nickel bump  30  completely and tightly so as to form an integrated protection layer. 
         [0037]    The top surface protection layer  40  and the sidewall protection layer  60  include following four different types of structure respectively. In practice, there are 16 possible combinations of the four bases taken two at a time. For example, the surface protection layer  40  with type one structure is paired with the sidewall protection layer  60  whose structure is type two, as shown in  FIG. 1 . The four different types of structure are as follows: 
         [0038]    Type one: a double-layer structure formed by an inner immersion gold (IG) layer and an outer electroless gold (EG) layer. The top surface protection layers  40  is formed by an inner IG layer  40   a  and an outer EG layer  40   b , as shown in  FIG. 1 . The sidewall protection layer  60  consists of an inner IG layer  60   a  and an outer EG layer  60   b , as shown in  FIG. 2 . First the exposed surface of the electroless nickel bump  30  (such as a top surface and a surrounding sidewall) is coated with an IG layer  40   a / 60   a  by immersion gold process. Then an EG layer  40   b / 60   b  is formed on an outer surface of the IG layer  40   a / 60   a . In this type, the thickness of the electroless nickel bump  30  is ranging from 2 μm to 14 μm. The thickness of the IG layer  40   a / 60   a  is about 0.01-0.05 μm while the thickness of the EG layer  40   b / 60   b  is about 0.5-2.0 μm. 
         [0039]    Type two: a single-layer structure formed by an immersion gold (IG) layer. Refer to  FIG. 3 , the top surface protection layers  40  is formed by an IG layer  40   a  and the sidewall protection layer  60  is formed by an IG layer  60   a . The IG layer  40   a / 60   a  is formed on the exposed surface of the electroless nickel bump  30  (such as a top surface and a surrounding sidewall) by immersion gold process. In the type two structure, the thickness of the electroless nickel bump  30  is ranging from 2 μm to 14 μm. The thickness of the IG layer  40   a / 60   a  is about 0.01 μm to 0.05 μm. 
         [0040]    Type three: a single-layer structure formed by an electroless silver (ES) layer. Refer to  FIG. 3 , the top surface protection layers  40  is formed by an ES layer  40   c  and the sidewall protection layer  60  is formed by an ES layer  60   c . The ES layer  40   c / 60   c  is formed on the exposed surface of the electroless nickel bump  30  (such as a top surface and a surrounding sidewall) by immersion silver process. In this structure, the thickness of the electroless nickel bump  30  is ranging from 2 μm to 14 μm. The thickness of the ES layer  40   c / 60   c  is about 0.5 μm to 2.0 μm. 
         [0041]    Type four: a double-layer structure formed by an inner electroless silver (ES) layer and an outer immersion gold (IG) layer. As shown in  FIG. 4 , the top surface protection layers  40  is formed by an inner ES layer  40   d  and an outer IG layer  40   e . The sidewall protection layer  60  is composed of an inner ES layer  60   d  and an outer IG layer  60   e . First an ES layer  40   d / 60   d  is coated on the exposed surface of the electroless nickel bump  30  (such as a top surface and a surrounding sidewall) by immersion silver process. Then an IG layer  40   e / 60   e  is formed on an outer surface of the ES layer  40   d / 60   d . In the type four structure, the thickness of the electroless nickel bump  30  is ranging from 2 μm to 14 μm. The thickness of the ES layer  40   d / 60   d  is about 0.5-2.0 μm while the thickness of the IG layer  40   e / 60   e  is about 0.01-0.05 μm. 
         [0042]    Refer to  FIG. 1 , the top surface protection layer  40  with the type one structure is used in combination with the sidewall protection layer  60  with the type two structure. Refer to  FIG. 2 , the top surface protection layer  40  with the type two structure is used together with the sidewall protection layer  60  with the type two structure. As shown in  FIG. 3 , the top surface protection layer  40  with the type two or type three structure is used in combination with the sidewall protection layer  60  with the type two or type three structure. While the sidewall protection layer  60   a  (with type two structure) being formed after the formation of the top surface protection layer  40   c  (with the type three structure) during manufacturing processes, an IG layer may be also formed on the outer surface of the top surface protection layer  40   c  so as to form the type four structure (not shown in  FIG. 3 ). The top surface protection layer  40  still achieves the expected functions. Refer to  FIG. 4 , the top surface protection layer  40  with the type four structure is used in combination with the sidewall protection layer  60  with the type four structure. 
         [0043]    Refer from  FIG. 5A  to  FIG. 5G , a manufacturing method of electroless nickel bumps of die pads includes a plurality of steps as follows. 
         [0044]    Refer to  FIG. 5A , provide a die  10  with a surface  11 . A plurality of die pads  12  is disposed on the surface and a first protection layer  13  with a plurality of openings  14  is formed on the surface  11 . The opening  14  is used for exposure of the die pad  12  correspondingly. The distance between the adjacent openings  14  is no more than 16 μm (μm=10 −6  m). 
         [0045]    Refer to  FIG. 5B , form a photoresist layer  50  on the first protection layer  13  and pattern the photoresist layer  50  so as to make the patterned photoresist layer  50  have a plurality of openings  51  corresponding to each die pad  12  and a part of the first protection layer  13  surrounding the die pad  12 . 
         [0046]    Refer to  FIG. 5C , form a catalyst layer  20  on a surface of each die pad  12  by a process selected from the group consisting of under bump metallization (UBM) and zincating. In an embodiment, the catalyst layer  20  made from zinc is produced by using 15-30% (w/w) aqueous solutions of zinc salts at 20-35 degrees Celsius for 10 seconds to 60 seconds. 
         [0047]    Refer to  FIG. 5D , form an electroless nickel bump  30  in each of the openings  51  by electroless nickel plating. In this embodiment, the thickness of the electroless nickel bump  30  is no less than 6 μm (the thickness ≧6 μm). In this embodiment, the electroless nickel bump  30  is formed under the condition arranged with the patterned photoresist layer  50 . The height of the electroless nickel bump  30  formed in this embodiment is generally larger than the height of the bump formed under the condition without the arrangement of the photoresist layer  50 . In this embodiment, the electroless nickel bump  30  is produced by deposition of 4-6.5 g/L aqueous solutions of nickel salts at 75-100° C. for 30 minutes to 75 minutes. The thickness of the electroless nickel bump  30  is 2-15 μm. 
         [0048]    Refer to  FIG. 5E , under the condition arranged with the patterned photoresist layer  50  shown in  FIG. 5D , form a top surface protection layer  40  on a top surface of each electroless nickel bump  30  by a process selected from the group consisting of immersion gold and immersion silver. The top surface protection layer  40  includes at least one protection layer, as  40   a ,  40   b  shown in  FIG. 1  and  FIG. 2 ,  40   a / 40   c  in  FIG. 3 , or  40   d ,  40   e  shown in  FIG. 4 . The top surface protection layer  40  is made from a material selected from the group consisting of immersion gold (IG) and electroless silver (ES). The top surface protection layer  40  can be one of the four types structure mentioned above such as the type one structure ( 40   a ,  40   b ) shown in  FIG. 1  and  FIG. 2 , the type two or the type three structure ( 40   a / 40   c ) in  FIG. 3  or the type four structure ( 40   d ,  40   e ) shown in  FIG. 4 . In the embodiment shown in  FIG. 5E  and  FIG. 5F , the top surface protection layer  40  is having the type one structure shown in  FIG. 1  and  FIG. 2 . 
         [0049]    Refer to  FIG. 5F , remove the photoresist layer  50  to expose the top surface protection layers  40 , the electroless nickel bumps  30  and a part of the first protection layers  13  not beyond the bumps  30 . 
         [0050]    Refer to  FIG. 5G , form a sidewall protection layer  60  on a surrounding sidewall of each electroless nickel bump  30  by a process sleeted from the group of immersion gold and immersion silver. Thus the manufacturing of the electroless nickel bumps of die pads  1  has been completed. The sidewall protection layer  60  includes at least one protection layer as  60   a  shown in  FIG. 1 ,  60   a ,  60   b  in  FIG. 2 ,  60   c  in  FIG. 3 , or  60   d ,  60   e  shown in  FIG. 4 . The sidewall protection layer  60  is made from a material selected from the group consisting of immersion gold (IG) and electroless silver (ES). The sidewall protection layer  60  can be one of the four types structure mentioned above such as the type two structure ( 60   a ) shown in  FIG. 1 , the type one structure ( 60   a ,  60   b ) shown in  FIG. 2 , the type three structure ( 60   c ) in  FIG. 3  or the type four structure ( 60   d ,  60   e ) shown in  FIG. 4 . In the embodiment shown in  FIG. 5G , the sidewall protection layer  60  is having the type two structure shown in  FIG. 1 . 
         [0051]    In the embodiment shown from  FIG. 5A  to  FIG. 5G , the top surface protection layers  40  and the sidewall protection layers  60  are produced by two separated processes. First top surface protection layers  40  are formed on top surface of the electroless nickel bumps  30  by a process selected from the group consisting of immersion gold and immersion silver. Then a sidewall protection layer  60  is formed on a surrounding sidewall of each electroless nickel bump  30  by a process selected from the group consisting of immersion gold and immersion silver after removing the photoresist layer  50  (as shown in  FIG. 5F ). Due to the separated processes, the structure type of the sidewall protection layers  60  can be different from that of the top surface protection layers  40 , as shown in  FIG. 1 . In this embodiment, the sidewall protection layer  60  is having type two structure—a single-layer structure formed by an immersion gold (IG) layer. Thus the cost is down and this doesn&#39;t affect the quality of the electroless nickel bumps of the die pads of the present invention. 
         [0052]    Moreover, refer from  FIG. 6A  to  FIG. 6F , another embodiment of a manufacturing method of electroless nickel bumps of die pads is revealed. The method includes a plurality of steps as follows. 
         [0053]    In this embodiment, step  6 A, step  6 B, step  6 C and step  6 D are the same as the step  5 A, step  5 B, step  5 C and step  5 D of the above embodiment. 
         [0054]    Refer to  FIG. 6E , remove the photoresist layer  50  to expose the electroless nickel bumps  30  including the top surface and the surrounding sidewall thereof, and a part of the first protection layers  13  not beyond the bumps  30 . 
         [0055]    Refer to  FIG. 6F , form an outer protection layer  40 ,  60  on a top surface and a surrounding sidewall of each electroless nickel bump  30  respectively at the same time by a process selected from the group of immersion gold and immersion silver. The top surface protection layer  40  on the top surface of each electroless nickel bump  30  and the sidewall protection layer  60  on the surrounding sidewall of each electroless nickel bump  30  are covered over the exposed surface of each electroless nickel bump  30  completely and tightly so as to form an integrated protection layer. Each outer protection layer  40 ,  60  includes at least one protection layer made from one material selected from immersion gold and electroless silver. 
         [0056]    In the embodiment shown from  FIG. 6A  to  FIG. 6F , the top surface protection layers  40  and the sidewall protection layers  60  are produced by the same process. That means they are formed on the exposed stop surface and the exposed surrounding sidewall of the electroless nickel bumps  30  by a process selected from the group consisting of immersion gold and immersion silver at the same time. Thus the structure type of the sidewall protection layers  60  is the same as that of the top surface protection layers  40 , as shown in  FIG. 2 ,  FIG. 3 , and  FIG. 4 . While the top surface protection layers  40  are with type one structure—a double-layer structure formed by an inner immersion gold (IG) layer and an outer electroless gold (EG) layer. Compared with the embodiment in  FIG. 1 , these embodiments have higher cost for materials. But this doesn&#39;t affect the quality of the electroless nickel bumps of the die pads of the present invention. 
         [0057]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.