Abstract:
The present invention relates to printed circuit boards (PCBs), and more particularly, to methods of forming high aspect ratio through holes and high precision stub removal in a printed circuit board (PCB). The high precision stub removal processes may be utilized in removing long stubs and short stubs. In the methods, multiple holes of varying diameter and depth are drilled from an upper and/or lower surface of the printed circuit board utilizing drills of different diameters.

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. §119 
       [0001]    The present Application for Patent claims priority to U.S. Provisional Application No. 61/930,456 entitled “METHODS OF FORMING HIGH ASPECT RATIO PLATED THROUGH HOLES AND HIGH PRECISION STUB REMOVAL IN A PRINTED CIRCUIT BOARD”, filed Jan. 22, 2014, assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to printed circuit boards (PCBs), and more particularly, to methods of forming high aspect ratio through holes and high precision stub removal in a printed circuit board (PCB). 
       BACKGROUND 
       [0003]    Consumers are increasingly demanding both faster and smaller electronic products. The use of PCBs has grown enormously as new electronic applications are marketed. A PCB is formed by laminating a plurality of conducting layers with one or more non-conducting layers. As the size of a PCB shrinks, the relative complexity of its electrical interconnections grows. 
         [0004]    A via structure is traditionally used to allow signals to travel between layers of a PCB. The plated via structure is a plated hole within the PCB that acts as a medium for the transmission of an electrical signal. For example, an electrical signal may travel through a trace on one layer of the PCB, through the plated via structure&#39;s conductive material, and then into a second trace on a different layer of the PCB. 
         [0005]    Unfortunately, due to limitations within the prior art, the plated via structure may be longer than necessary to perform the function of electrical connectivity. For example, the plated via structure may extend completely through the PCB but only connect two traces on two proximate adjacent layers. As a result, one or more stubs may be formed. A stub is excessive conductive material within the plated via structure which is not necessary to transport the electrical signal. 
         [0006]    When a high speed signal is transmitted through the plated via structure, a “stub effect” may distort the signal. The stub effect is a result of the useless excess conductive material present within the plated via structure. The stub effect occurs when a portion of the signal is diverted away from the trace connections and into one or more stubs of the plated via structure. The portion of the signal may be reflected from the end of the stub back toward the trace connections after some delay. This delayed reflection may interfere with signal integrity and increase, for example, the bit error rate of the signal. The degenerating effect of the stub effect may increase with the length of the stub. 
         [0007]      FIGS. 1A-1E  illustrate the typical stages of forming back-drilled hole in a printed circuit board (PCB). As shown, the PCB  100  includes stacked material insulator layers (typically laminates and prepregs)  104   a ,  104   b  . . .  104   f  separating five internal layers  105   a ,  105   b  . . .  105   e  and two external layers  108   a  and  108   b . The internal layers  105   a ,  105   b  and  105   e  are signal layers and the internal layers  105   c  and  105   d  are plane layers. The PCB  100  has an upper surface  110   a  and an opposing lower surface  110   b.    
         [0008]    To form the plated through hole  101  as illustrated in  FIG. 1E , a first hole  102  having a first diameter d 1  is drilled through the PCB  100  (see  FIG. 1A ). Next, a second hole  103  having a second diameter d 2  is drilled concentrically around and through a predetermined depth of the first hole  102  (see  FIG. 1B ). Then, the walls of the remaining portion of the first hole  102  and the walls of the second hole  103  are plated with a thin layer of a conductive material  106  (e.g., copper). (See  FIG. 1C ). 
         [0009]    Next, a drill  112  having a diameter d 3  is used to back-drill and form a third hole  104  concentrically around and through the remaining portion of the plated first hole  402  starting from the lower surface  110   b  of the printed circuit board  100  (see  FIG. 1D ). Then the drill  112  is removed from the printed circuit board  100  which now has the via  101  formed therein that includes a first through hole  107  (e.g., plated second hole  103 ) and a back-drilled hole  109  (e.g., third hole  104 ) (see  FIG. 1E ). 
         [0010]    However, when back-drilling the drill bit has no pressure and it pushes up against the traces on the top and as a result fluctuating is created as well as peel off of the conductive plating occurs. (See  FIGS. 1D-1E ) Consequently, there is a need for improved methods for removing stubs when forming plated through holes in a PCB. 
       SUMMARY 
       [0011]    The following presents a simplified summary of one or more implementations in order to provide a basic understanding of some implementations. This summary is not an extensive overview of all contemplated implementations, and is intended to neither identify key or critical elements of all implementations nor delineate the scope of any or all implementations. Its sole purpose is to present some concepts of one or more implementations in a simplified form as a prelude to the more detailed description that is presented later. 
         [0012]    According to one aspect, a method for forming a plated through hole in a printed circuit board is provided. The method includes drilling a first hole having a first diameter through an upper surface of the printed circuit board; drilling a second hole having a second diameter through the first hole to a lower surface of the printed circuit board, the lower surface opposite the upper surface; plating the first hole and the second hole with a conductive material; and drilling a third hole having a third diameter through the first hole and the second hole. The printed circuit board includes a plurality of plated through holes formed therein. 
         [0013]    In one example, the first hole is drilled a predetermined depth and the predetermined depth is shorter than half the vertical distance of the printed circuit board. The second diameter is smaller than the first diameter, and the third diameter is larger than the second diameter and smaller than the first diameter. The third hole is drilled from the upper surface to the lower surface of the printed circuit board. 
         [0014]    In another example, the second diameter is larger than the first diameter, and the third diameter is larger than the second diameter and smaller than the first diameter. 
         [0015]    In yet another example, the second diameter is smaller than the first diameter; a top of the third hole to a bottom of the second hole remains undrilled; and the third diameter is larger than the second diameter. 
         [0016]    In yet another example, the method further includes adding copper to any remaining conductive surface by electrolytic plating. 
         [0017]    In yet another example, the second diameter is smaller than the first diameter; a top of the third hole to a bottom of the first hole remains undrilled; and the third diameter is larger than the second diameter. The method further includes adding copper to any remaining conductive surface by electrolytic plating. 
         [0018]    In yet another example, the method further includes embedding a first conductive material in the printed circuit board 
         [0019]    According to another aspect, a method for forming a plated through hole in a printed circuit board is provided. The method includes drilling a first hole having a first diameter from a lower surface of the printed circuit board; drilling a second hole having a second diameter through an upper surface of the printed circuit board to the first hole of the printed circuit board, the upper surface opposite the lower surface; plating the first hole and the second hole with a conductive material; and drilling a third hole having a third diameter through the first hole and the second hole. The printed circuit board has a plurality of plated through holes formed therein. 
         [0020]    According to one example, the second hole is drilled a predetermined depth; and the predetermined depth is shorter than half the vertical distance of the PCB. The second diameter is larger than the first diameter; and the third diameter is larger than the second diameter and smaller than the first diameter. The third hole is drilled from the upper surface to the lower surface of the printed circuit board. 
         [0021]    According to another example, the second diameter is larger than the first diameter; and the third diameter is smaller than the second diameter and larger than the first diameter. 
         [0022]    According to yet another example, the second diameter is larger than the first diameter; a top of the third hole to a bottom of the second hole remains undrilled; and the third diameter is larger than the first diameter. 
         [0023]    According to yet another example, the method further includes adding copper to any remaining conductive surface by electrolytic plating. 
         [0024]    According to yet another example, the first hole is drilled a predetermined depth; and the predetermined depth is shorter than half the vertical distance of the printed circuit board. The method further includes plating the third hole with a conductive material forming a plated through hole; wherein the second hole is drilled to a top of the first hole; and the third diameter is larger than the first diameter and the second diameter. 
         [0025]    According to yet another example, a method for forming a plated through hole in a printed circuit board, including a first surface and an opposing second surface, is provided. The method includes embedding a first conductive material in the printed circuit board; drilling a first hole having a first diameter from a first surface of the printed circuit board to a first predetermined depth; drilling a second hole having a second diameter through the first hole to a second predetermined depth, plating the first hole and the second hole with a second conductive material; and drilling a third hole having a third diameter through the first hole and the second hole. The printed circuit board has a plurality of plated through holes formed therein; and wherein the conductive material is copper. 
         [0026]    According to one example, the first surface is an upper surface and the second surface is a lower surface; the first predetermined depth is from the upper surface to the embedded first conductive material; the second predetermined depth is from the first surface to the second surface; wherein second diameter is smaller than the first diameter; and the third diameter is larger than the second diameter and smaller than the first diameter. 
         [0027]    According to another example, the first surface is a lower surface and the second surface is an upper surface; the first predetermined depth is from the lower surface through the embedded first conductive material; the second predetermined depth is from the first surface to the first hole of the printed circuit board; and the second diameter is smaller than the first diameter; and the third diameter is larger than the second diameter and smaller than the first diameter. 
         [0028]    According to yet another example, the first surface is a lower surface and the second surface is an upper surface; the first predetermined depth is from the lower surface through the embedded first conductive material; the second predetermined depth is from the first surface to the first hole of the printed circuit board; and the second diameter is smaller than the first diameter; and the third diameter is larger than the first diameter and smaller than the second diameter. 
         [0029]    According to yet another example, the first surface is a lower surface and the second surface is an upper surface; the first predetermined depth is from the lower surface through the embedded first conductive material; the second predetermined depth is from the upper surface to the printed circuit board through the embedded first conductive material; the second diameter is smaller than the first diameter; and the third diameter is smaller than the second diameter and larger than the second diameter. 
         [0030]    According to yet another example, the method includes adding copper to any remaining conductive surface by electrolytic plating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIGS. 1A-1E  illustrate the typical stages of forming back-drilled hole in a PCB. 
           [0032]    FIGS.  2 A 1 ,  2 A 2  and  2 B- 2 D illustrate the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board (PCB), according to one aspect. 
           [0033]      FIGS. 3A-3G  are illustrations of the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board (PCB), according to one aspect. 
           [0034]      FIGS. 4A-4D  are illustrations of the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board, according to one aspect. 
           [0035]      FIGS. 5A-5D  illustrate the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board, according to one aspect. 
           [0036]      FIG. 6  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. 
           [0037]      FIG. 7  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. 
           [0038]      FIG. 8  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. 
           [0039]      FIG. 9  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. 
           [0040]      FIGS. 10A-10G  are illustrations of the different fabrication stages for forming a plated through hole (or via) and removal of a short stub in a printed circuit board (PCB), according to one aspect. 
           [0041]    FIGS.  11 A 1 ,  11 A 2 ,  11 B- 1 ,  11 B- 2 ,  11 C and  11 D are illustrations of the different fabrication stages for forming a plated through hole (or via) and removal of a short stub in a printed circuit board (PCB), according to one aspect. 
           [0042]      FIG. 12  illustrates a method for forming a plated through hole in a laminated PCB, according to one aspect. 
           [0043]      FIG. 13  illustrates a method for forming a plated through hole in a laminated PCB, according to one aspect. 
           [0044]      FIG. 14  illustrates a method for forming a plated through hole in a laminated PCB, according to one aspect. 
           [0045]      FIG. 15  illustrates a method for forming a plated through hole in a laminated PCB, according to one aspect. 
           [0046]      FIGS. 16A-16H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
           [0047]      FIGS. 17A-17H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) and adjacent conductive material filled prepreg for forming a via without a stub, according to one aspect. 
           [0048]      FIGS. 18A-18H  illustrate of the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg between conductive material filled cores; and having a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
           [0049]      FIGS. 19A-19G  illustrate different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg and solid core as well as having a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
           [0050]      FIGS. 20A-20H  illustrate different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg between a conductive material filled core and a solid core; and a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
           [0051]      FIGS. 21A-21H  illustrate different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and multiple conductive material filled cores having a connecting inner layer trace (target layer) and adjacent conductive material filled prepregs for forming a via without a stub, according to one aspect. 
           [0052]      FIGS. 22A-22G  are illustrations of the different fabrication stages for forming a plated through hole (or via) having a high aspect ratio in a printed circuit board (PCB). 
           [0053]      FIG. 23  illustrates a method for forming a plated through hole, having a high aspect ratio, in a PCB, according to one aspect. 
           [0054]      FIGS. 24A-24D  illustrate a method of forming a plated through hole, having a high aspect ratio, in a printed circuit board, according to one aspect. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]    While the present disclosure provides methods for forming plated through holes in multi-layer printed circuit boards, the present disclosure is not restricted to us in PCBs. A multilayer PCB can be a package substrate, a motherboard, a line card, a puddle card, a backplane, a midplane, a flex or rigid flex circuit. A via structure can be a plated through hole (PTH) used for transmitting electrical signals from one conducting layer to another. A plated via structure can also be a component mounting hole for electrically connecting an electrical component to other electrical components on the PCB. 
       Removal of Long Stub 
       [0056]    FIGS.  2 A 1 ,  2 A 2  and  2 B- 2 D illustrate the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board (PCB), according to one aspect. As described below, the final drill direction prevents the peeling up of the cutting edge of the plated corner. 
         [0057]    The location where a signal is transmitted through the PTH, or where a first layer  202  is connected to a second layer  204  in the PCB  200 , may be represented as a target layer (T). According to one aspect, the first layer  202  may include a hole  206  drilled using a long depth drill while the second layer  204  may include an upper hole  203  drilled using a short depth drill. The long depth drill has a length that is less than or equal to the distance from the bottom of the first layer  202  to the target layer T prior to plating. 
         [0058]    As the relative drill accuracy to PCB layers can depend on the drill depth length, drilling a hole having shorter depth provides better accuracy then drilling a hole having a longer depth. As shown in  FIG. 2A-1 , the upper hole  203  is formed using the short depth drill and has a diameter of d 1  and a depth length L 1 , such that the bottom of the upper hole  203   a  is below the target layer T. In other words, the short depth drill passes through the target layer T when drilling from the second layer  204  downward. 
         [0059]    Next, as shown in  FIG. 2B , the long depth drill may be used to drill a lower hole  206  up to and through the upper hole  203  forming a complete hole through the PCB, or the long depth drill is performed downward from hole  203  to the other PCB surface. The lower hole  206  has a diameter d 2  and a length L 2  where d 2  is less than d 1  and L 2  is greater than L 1 . 
         [0060]    Alternatively, as shown in  FIG. 2A-2 , the lower hole  206  may be drilled first, using the long depth drill, up to the Target Layer T. Next, the small depth drill may be used to drill the upper hole  203  such that it connects with the lower hole  206  at or below the target layer T. (See  FIG. 2B ) 
         [0061]    After the dual diameter hole (i.e. the upper hole  203  having a diameter d 1  and lower hole  206  having a diameter d 2 ) has been formed, the dual diameter hole may then be plated by a conductive material  207 . (See  FIG. 2C ) Next, a middle drill, having a diameter d 3 , where d 3  is greater than d 2  but less than d 1 . may be used to remove plated conductive material from the surface of the lower hole  206  by drilling down through the upper hole  203  and drilling the conductive material on the lower hole  206 . 
         [0062]    Referring to  FIGS. 3A-3G , different fabrication stages for forming a plated through hole (or via)  300  ( FIG. 3G ) in a printed circuit board (PCB)  301  are illustrated. The PCB  301  has an upper surface  301   a  and an opposing lower surface  301   b . The through hole  300  has an upper conductive portion  302  and a lower non-conductive portion  304 . 
         [0063]    A three (3) drill step process may be used to form the plated through hole  300 . In the first step, a first hole  306  is drilled, from the upper surface  301   a  of the PCB, using a first drill  308  having a first diameter d 1 . The first hole  306  is drilled to a predetermined depth, such as just past a target layer T in the PCB  301 . (See  FIGS. 3A and 3B ) 
         [0064]    Next, a second hole  310  may be drilled, using a second drill  312  having a second diameter d 2 , where d 2  is smaller than d 1 . (See  FIGS. 3C , and  3 D) The interior surface of the holes  306  and  310  may then be plated  311  with a conductive material. ( FIGS. 3E and 3F ) 
         [0065]    Referring to  FIG. 3F , after the first and second holes  306  and  310  are plated  311 , a third drill  314 , having a third diameter d 3 , may pass (drill) through the first and second holes  306  and  310 . The third diameter d 3  is larger than the second diameter d 2  but smaller than the first diameter d 1 . As the third drill  314  has a larger diameter then the second hole  310  but a smaller diameter than the first hole  306 , the conductive material  311  in the first hole  306  may be left intact but the conductive material on the second hole  310  is removed, i.e. the stub area is removed. As a result, the through hole  300 , as shown in  FIG. 3G , has an upper conductive portion  302  and a lower non-conductive portion  304 . 
         [0066]    Alternatively, the sequence of the first and second drill processes described above can be changed. In the first step, a first hole  310  may be drilled, from the lower surface  301   b  of the PCB, using a first drill  312  having a first diameter d 1 . The first hole  310  may be drilled to a predetermined depth, such as just before the target layer in PCB  301  or drilled through the entire PCB  301 , such as shown in  FIG. 2A-2 . 
         [0067]    Next, a second hole  306  may be drilled, from the upper surface  301   a  of the PCB, using a second drill  308  having a second diameter d 2 , where d 2  is larger than d 1  (See  FIG. 2B ) The interior surface of the holes  306  and  310  may then be plated  311  with a conductive material. ( FIGS. 3E and 3F ) 
         [0068]    Referring to  FIG. 3F , after the first and second holes  306  and  310  are plated  311 , a third drill  314 , having a third diameter d 3 , may pass (or drill) through the holes first and second  306  and  310 . The third diameter may be larger than the second diameter d 2  but smaller than the first diameter d 1 . As the third drill  314  has a larger diameter then the second hole  310  but a smaller diameter than the first hole  306 , the conductive material  311  in the first hole  306  may be left intact while the conductive material on the second hole  310  is removed, i.e. the stub area is removed. As a result, the through hole  300 , as shown in  FIG. 3G , may include an upper conductive portion  302  and a lower non-conductive portion  304 . 
         [0069]      FIGS. 4A-4D  illustrate the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board, according to one aspect. The location where a signal goes through the PTH, or where a first layer  402  is connected to a second layer  404  in the PCB  400 , may be represented as a target layer (T). According to one aspect, the first layer  402  may include a first hole  406 , having a diameter d 1 , that is drilled through the first and second layers  402 ,  404  and a second hole  408 , having a diameter d 2 , where d 2  is greater than d 1 , may be drilled from the top surface of the board  400  to slightly below the target conductive layer T. A seeding conductive material  410 , such as electroless copper plating, is applied to the first hole  406  and the second hole  408 . A third hole  412 , having a diameter d 3 , which is greater than d 1  but less than d 2 , is back drilled to slightly below the target layer T. As d 3  is greater than d 1 , the seeding conductive material  410  in the second hole  408  is removed upon drilling the third hole  412 . Next, electrolytic plating  414  is then applied to the second hole  408 . 
         [0070]      FIGS. 5A-5D  illustrate the different fabrication stages for forming a plated through hole (or via) and removal of a long stub in a printed circuit board, according to one aspect. The location where a signal goes through the PTH, or where a first layer  502  is connected to a second layer  504  in the PCB  500 , is represented as a target conductive layer (T). According to one aspect, the first layer  502  may include a first hole  506 , having a diameter d 1 , drilled through the first and second layers  502 ,  504 . A seeding conductive material  508 , such as electroless copper plating, is then applied to the first hole  506 . Next, a second hole  510 , having a diameter d 2 , where d 2  is greater than d 1 , is back drilled to slightly below the target conductive layer T. After back drilling the second hole  510 , electrolytic plating  512  is then applied to the first hole  506 . 
         [0071]      FIG. 6  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. In the method, a first hole, having a first diameter, may be drilled, from an upper surface of the PCB, a predetermined depth in a printed circuit board using a first drill having a first diameter  602 . For example, the predetermined depth may be half way through the PCB. Next, a second hole, having a second diameter, may be drilled through the remaining portion of the PCB to the lower surface of the PCB  604  creating or forming a through hole. The second diameter may be smaller than the first diameter. Next, the first hole and the second hole may be plated with a conductive material  606 . Next, a third hole having a third diameter may be drilled through the first hole and the second hole from the upper surface to the lower surface of the PCB, the third diameter is larger than the second diameter and smaller than the first diameter  608 . As the third diameter is larger than the second diameter and smaller than the first diameter, when the third hole is drilled the plated material may be removed from the inner surface of the second hole. That is, the stub may be removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion is formed. 
         [0072]    In another aspect,  FIG. 7  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. In the method, a first hole, having a first diameter, may be drilled, from a lower surface of the PCB, a predetermined depth in a printed circuit board using a first drill having a first diameter  702 . For example, the predetermined depth may be half way through the PCB. Next, a second hole having a second diameter may be drilled through an upper surface of the PCB to the first hole of the PCB  704 . The first diameter is smaller than the second diameter. Next, the first hole and the second hole may be plated with a conductive material  706 . Next, a third hole having a third diameter may be drilled through the first hole and the second hole from the upper surface to the lower surface of the PCB, the third diameter is smaller than the second diameter and larger than the first diameter  708 . As the third diameter is smaller than the second diameter and larger than the first diameter, when the third hole is drilled the plated material may be removed from the inner surface of the first hole. That is, the stub is removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion is formed. 
         [0073]    In yet another aspect,  FIG. 8  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. A first hole, having a first diameter, may be drilled, from an upper surface of the PCB, a predetermined depth in a printed circuit board using a first drill having a first diameter  802 . For example, the predetermined depth may be half way through the PCB. Next, a second hole having a second diameter may be drilled through the first hole to the lower surface of the PCB  804 . The second diameter is smaller than the first diameter. Next, the first hole and the second hole may be plated with a thin conductive material such as electroless plating catalyst or electroless copper  806 . Next, a third hole having a third diameter may be drilled through the first hole and the second hole of the PCB, the third diameter is larger than the second diameter and smaller than the first diameter  808 . A conductor, such as copper, may then be added to the remaining conductive area by electrolytic plating  810 . As the third diameter is larger than the second diameter and smaller than the first diameter, the third hole may prevent plating up of the conductive material so as to remove the thin base conductor, for electrolytic plating, from the inner surface of the first hole. That is, the stub may be prevented. As a result, a through hole having an upper conductive portion and a lower non-conductive portion may be formed. 
         [0074]    In yet another aspect,  FIG. 9  illustrates a method for forming a plated through hole and removing a long stub in a laminated PCB, according to one aspect. In the method, a first hole, having a first diameter, may be drilled, from a lower surface of the PCB, a predetermined depth in a printed circuit board using a first drill having a first diameter  902 . For example, the predetermined depth may be half way through the PCB. Next, a second hole having a second diameter may be drilled through an upper surface of the PCB to the first hole of the PCB  904 . The second diameter is larger than the first diameter. Next, the first hole and the second hole may be plated with a thin conductive material such as electroless plating catalyst or electroless copper  906 . Next, a third hole having a third diameter may be drilled through the first hole and the second hole of the PCB, the third diameter may be larger than the first diameter and smaller than the second diameter  908 . A conductor, such as copper, may then be added to the remaining conductive area by electrolytic plating  910 . As the third diameter is larger than the first diameter and smaller than the second diameter, the third hole may prevent plating up of the conductive material so as to remove the thin base conductor, for electrolytic plating, from the inner surface of the first hole. That is, the stub generation may be prevented. As a result, a through hole having an upper conductive portion and a lower non-conductive portion may be formed. 
       Removal of Short Stub 
       [0075]    Referring to  FIGS. 10A-10G , the different fabrication stages for forming a plated through hole (or via)  1000  in a laminated printed circuit board (PCB)  1001  are illustrated. The PCB  1001  may have an upper surface  1001   a  and a lower surface  1001   b . The through hole  1000  may have an upper conductive portion  1002  and a lower non-conductive portion  1004 . 
         [0076]    As described below, a three (3) drill step process may be used to form the plated through hole. In the first step, a first hole  1006  may be drilled, i.e. from the lower surface  1001   b  of the PCB, to a predetermined depth using a first drill  1008  having a first diameter d 1 . The first hole  1006  may be drilled just past the target layer in the PCB  1001 . (See  FIGS. 10A and 10B ) 
         [0077]    Next, a second hole  1010  may be drilled, from the upper surface  1001   a  to the top of the first hole  1006 , using a second drill  1012  having a second diameter d 2 , where d 2  is larger than d 1 . (See  FIGS. 10C and 10D ) The interior surface of the holes  1006  and  1010  may be then plated with a conductive material  1013 . ( FIGS. 10E and 10F ) 
         [0078]    Referring to  FIG. 10F , after the first and second holes  1006  and  1000  are plated  1011 , a third drill  1014 , having a third diameter d 3 , may pass (drill) through the first and second holes  1006  and  1010 . The third diameter is larger than the first diameter d 1 . As the third drill  1014  has a larger diameter then the first hole  1006  but a smaller diameter than the second hole  1010 , the conductive material in the second hole  1010  may be left intact but the majority of the conductive material on the first hole  1006  is removed, i.e. the stub area is removed. As shown in  FIG. 10G , the third drill  1014  may not be drilled all the way through the plated first hole  1006  leaving a small portion or small stub  1016  of the conductive material on the inner surface of the first hole  1006 . A small stub  1016  may remain because when drilling upwardly from the lower surface  1001   b  of the PCB, the drill bit has no pressure and creates fluctuating as well as copper peel off. Also, the short depth drill has higher depth accuracy than the long depth drill, so controlling the stub length by using a shorter depth drill may provide shorter stub length than when using a longer depth drill. For instance, this process may provide a shorter stub than when a third drill is drilled from top hole  1010  to bottom hole  1006  as in like  FIG. 2  and  FIG. 3  which is controlled stub length by the second drill. As a result, the through hole  1000 , as shown in  FIG. 10G , may have an upper conductive portion  1002 , a lower non-conductive portion  1004  and a small stub  1016 . ( FIG. 10G ) 
         [0079]    Alternatively, in the first step, a first hole  1010  may be drilled, i.e. from the upper surface  1001   a  of the PCB, to a predetermined depth using the first drill  1012  having a first diameter d 1 . The first hole  1010  may be drilled to just before the target layer in the PCB  1001 . Next, a second hole  1006  may be drilled, from the lower surface  1001   b  to the bottom of the first hole  1010 , using a second drill  1008  having a second diameter d 2 , where d 2  is smaller than d 1 . (See  FIG. 10D  and The interior surface of the holes  1006  and  1010  may then be plated with a conductive material. ( FIGS. 10E and 10F ) 
         [0080]    Referring to  FIG. 10F , after the first and second holes  1010  and  1006  are plated  1011 , a third drill  1014 , having a third diameter d 3 , may pass (drill) through the first and second holes  1006  and  1010 . The third diameter is larger than the second diameter d 2 . As the third drill  1014  has a larger diameter then the second hole  1006  but a smaller diameter than the hole  1010 , the conductive material in the first hole  1010  may be left intact but the majority of the conductive material on the first hole  1006  may be removed, i.e. the stub area is removed. As shown in  FIG. 10G , the third drill  1014  is not drilled all the way through the plated first hole  1006  leaving a small portion or small stub  1016  of the plating resist on the inner surface of the first hole  1006 . A small stub  1016  may remain because when drilling upwardly from the lower surface  1001   b  of the PCB, the drill bit has no pressure and creates fluctuating as well as copper peel off. Also, short depth drill has higher depth accuracy than long depth drill, so controlling stub length by shorter depth drill may provide shorter stub length than longer depth drill. For instance, this process may provide a shorter stub than when drilled using the third drill from the second (or top) hole  1010  to the first (or bottom) hole  1006  as in  FIG. 2  and  FIG. 3  where the stub length is controlled by a long depth second drill. As a result, the through hole  1000 , as shown in  FIG. 10G , may have an upper conductive portion  1002 , a lower non-conductive portion  1004  and a small stub  1016 . (See  FIG. 10G ) 
         [0081]    FIGS.  11 A 1 ,  11 A 2  and  11 B- 11 G are illustrations of the different fabrication stages for forming a plated through hole (or via)  1100  and removal of a short stub in a printed circuit board (PCB)  1101 , according to one aspect. The printed circuit board has an upper surface  1101   a  and an opposing lower surface  1101   b.    
         [0082]    The location where a signal goes through the PTH, or where a first layer  1102  is connected to a second layer  1104  in the PCB  1100 , may be represented as a target layer (T). According to one aspect, the first layer  1102  may include an upper hole  1108  drilled using a long depth drill while the second layer  1104  may include a lower hole  1106  drilled using a short depth drill. The long depth drill has a length that is less than or equal to the distance from the bottom of the first layer  1102  to the target layer T prior to plating. 
         [0083]    As shown in  FIG. 11A-1 , the lower hole  1106  may be formed using a short depth drill having a diameter of d 1 , such that the top of the lower hole  1106  may be just above the target layer T. In other words, the short depth drill passes through the target layer T when drilling from the first layer  1104  upward. Next, the upper hole  1108  may be drilled, from the upper surface  1101   a  to the top of the lower hole  1106  using a long depth drill having a diameter of d 2 , where d 2  is larger than d 1 . (See  FIG. 11B-1 ) The interior surface of the upper and lower holes  1108  and  1106  may be then plated  1112  with a conductive material. ( FIG. 11C ) 
         [0084]    After the upper and lower holes  1110  and  1106  are plated  1112 , a third hole  1114 , having a diameter d 3 , which is greater than d 1  but less than d 2 , is back drilled to slightly below the target layer T. As d 3  is greater than d 1 , the conductive material  1112  in the upper hole  1108  may be left intact but the majority of the conductive material on the lower hole  1106  may be removed, i.e. the stub area is removed. As shown in  FIG. 11D , the third hole  1114  is not drilled, all the way through the plated lower hole  1106  leaving a small portion or small stub  1116  of the plating resist on the inner surface of the first hole  1106 . A small stub  1116  may remain because when drilling upwardly from the lower surface  1101   b  of the PCB, the drill bit has no pressure and creates fluctuating as well as copper peel off. Also, a short depth drill has higher depth accuracy than long depth drill, so controlling stub length by shorter depth drill may provide shorter stub length than longer depth drill. For instance, this process may provide a shorter stub than when drilled with the third drill from upper hole  1110  to lower hole  1106  as in  FIG. 2  and  FIG. 3  where the stub length is controlled by a long depth second drill. As a result, the through hole  1100 , as shown in  FIG. 11D  may have an upper conductive portion, a lower non-conductive portion and a small stub  1116 . 
         [0085]    Alternatively, as shown in  FIG. 11A-2 , the upper hole  1108  may be drilled first, using the long depth drill, down to the Target Layer T. Next, the small depth drill may be used to drill the upper hole  1108  such that it connects with the lower hole  1106  at or below the target layer T. Next, the lower hole  1106  may be drilled, from the lower surface  1101   b  to the bottom of the lower hole  1106  using a short depth drill having a diameter of d 2 , where d 2  is smaller than d 1 . (See  FIG. 12B-2 ) The interior surface of the upper and lower holes  1108  and  1106  may be then plated  1112  with a conductive material. ( FIG. 11C ) 
         [0086]    After the upper and lower holes  1110  and  1106  are plated  1112 , a third hole  1114 , having a diameter d 3 , which is greater than d 1  but less than d 2 , is back drilled to slightly below the target layer T. As d 3  is greater than d 1 , the conductive material  1112  in the upper hole  1108  may be left intact but the majority of the conductive material on the lower hole  1106  may be removed, i.e. the stub area is removed. As shown in  FIG. 11D , the third hole  1114  is not drilled, all the way through the plated lower hole  1106  leaving a small portion or small stub  1116  of the plating resist on the inner surface of the first hole  1106 . A small stub  1116  may remain because when drilling upwardly from the lower surface  1101   b  of the PCB, the drill bit has no pressure and creates fluctuating as well as copper peel off. Also, a short depth drill has higher depth accuracy than long depth drill, so controlling stub length by shorter depth drill may provide shorter stub length than longer depth drill. For instance, this process may provide a shorter stub than when drilled with the third drill from upper hole  1110  to lower hole  1106  as in  FIG. 2  and  FIG. 3  where the stub length is controlled by a long depth second drill. As a result, the through hole  1100 , as shown in  FIG. 11D  may have an upper conductive portion, a lower non-conductive portion and a small stub  1116 . 
         [0087]      FIG. 12  illustrates a method for forming a plated through hole in a laminated PCB, according to one aspect. In the method, a first hole, having a first diameter, may be drilled from a lower surface of the PCB to a predetermined depth using a first drill having a first diameter  1202 . For example, the predetermined depth may be half way through the PCB. Next, a second hole may be drilled from an upper surface of the PCB to a top of the first hole using a second drill having a second diameter, where the second diameter is larger than the first diameter  1204 . The first hole and the second hole may then be plated with a conductive material  1206 . Next, a third hole, having a third diameter, may be drilled to the first hole from the lower surface of the PCB using a third drill, where the third diameter is larger than the first diameter  1208 . As the third diameter is larger than the first diameter and smaller than the second diameter, when the third hole is drilled the plated material may be removed from the inner surface of the first hole. That is, the stub is removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion may be formed. 
         [0088]    In another aspect, as illustrated in  FIG. 13 , a first hole, having a first diameter, may be drilled from an upper surface of the PCB to a predetermined depth using a first drill having a first diameter  1302 . For example, the predetermined depth may be half way through the PCB. Next, a second hole may be drilled through the first hole of the PCB to a bottom of the first hole using a second drill having a second diameter, where the second diameter is smaller than the first diameter  1304 . The first hole and the second hole may then be plated with a conductive material  1306 . Next, a third hole, having a third diameter, may be drilled to the first hole from the lower surface of the PCB using a third drill, where the third diameter is larger than the second diameter  1308 . As the third diameter is larger than the second diameter, when the third hole is drilled the plated material is removed from the inner surface of the second hole. That is, the stub is removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion may be formed. 
         [0089]    In another aspect, as illustrated in  FIG. 14 , a first hole may be drilled from a lower surface of the PCB to a predetermined depth using a first drill having a first diameter  1402 . For example, the predetermined depth may be half way through the PCB. Next, a second hole may be drilled from an upper surface of the PCB to a top or upper end of the first hole using a second drill having a second diameter, where the second diameter is larger than the first diameter  1404 . The first hole and the second hole may then be plated with a thin conductive material such as electroless plating catalyst or electroless copper  1406 . Next, a third hole, having a third diameter, may be drilled to the first hole from the lower surface of the PCB using a third drill, where the third diameter may be larger than the first diameter  1408 . A conductive material, such as copper, may then be added to the remaining conductive area by electrolytic plating  1410 . As the third diameter is larger than the first diameter, when the third hole is drilled the plated material may be removed from the inner surface of the first hole. That is, the stub is removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion may be formed. 
         [0090]    In another aspect, as illustrated in  FIG. 15 , a first hole is drilled from an upper surface of the PCB to a predetermined depth using a first drill having a first diameter  1502 . For example, the predetermined depth is half way through the PCB. Next, a second hole is drilled through the first hole to a bottom of the first hole using a second drill having a second diameter, where the second diameter is smaller than the first diameter  1504 . The first hole and the second holes are then plated with a thing conductive material such as electroless plating catalyst or electroless copper  1506 . Next, a third hole, having a third diameter, is drilled to the first hole from the lower surface of the PCB using a third drill, where the third diameter is larger than the second diameter  1508 . Conductive material, such as copper, is then added to the remaining conductive area by electrolytic plating  1510 . As the third diameter is larger than the first diameter and smaller than the second diameter, when the third hole is drilled the plated material is removed from the inner surface of the second hole. That is, the stub is removed. As a result, a through hole having an upper conductive portion and a lower non-conductive portion is formed. 
         [0000]    Forming a Via without a Stub 
         [0091]    In another aspect, the present disclosure provides a method of using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) for forming a via without a stub.  FIGS. 16A-16H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
         [0092]    In the method, four different drill sizes may be utilized to drill through an upper surface  1600   a  and lower surface  1600   b  of a PCB  1600  passing through embedded conductive material in the PCB  1600 . First, a first hole  1602  is drilled in a core  1604  having a diameter of d 1  (See  FIGS. 16A-16D ) and is filled with conductive material  1606 , such as conductive paste or plated copper. Next, a second hole  1608 , where the second hole  1608  is larger than the first hole  1602 , is drilled with diameter of d 2  after lamination until reaching the embedded conductive material  1606 . Next, a third hole  1610 , having a diameter of d 3 , is drilled by drilling through the second hole  1608 , where diameter d 3  is smaller than d 1  and d 2 . Next, the second and third holes  1608  and  1610  are plated with a conductive material  1612  such as copper. Next, the plated conductive material is removed from the surface of the third hole  1610  by drilling through the second hole  1608  forming a fourth  1614  having a diameter of d 4 , where d 4  is larger than d 3  and smaller than d 1  and d 2 . (See  FIGS. 16E-16H ) 
         [0093]    In another aspect, the present disclosure provides a method of using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) and adjacent conductive material filled prepreg for forming a via without a stub.  FIGS. 17A-17H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) and adjacent conductive material filled prepreg for forming a via without a stub, according to one aspect. 
         [0094]    In the method, four different drill sizes may be utilized to drill through an upper surface  1700   a  and lower surface  1700   b  of a PCB  1700  passing through embedded conductive material in the PCB  1700 . First, a first hole  1702  is drilled in a core  1704  and prepreg  1706  having a diameter of d 1  and filled with conductive material  1708 , such as conductive paste or plated copper. Next, a second hole  1710 , where the second hole  1710  is larger than the first hole  1702 , is drilled having a diameter of d 2  after lamination, until reaching the embedded conductive material  1708 . Next, a third hole  1712 , where the third hole is smaller than the second hole  1710 , having a diameter d 3  is drilled by drilling through the second hole  1710 , where d 3  is smaller than d 1  and d 2 . Next, the second hole  1710  and the third hole  1712  are plated over by a conductive material  1714  such as copper. Next, the plated conductive material is removed from the surface of the third hole  1712  by drilling through the second hole  1710  forming a fourth hole  1716  having a diameter of d 4 , where d 4  is larger than d 3  and smaller than d 1  and d 2 . (See  FIGS. 17E-17H ) 
         [0095]    In another aspect, the present disclosure provides a method of using different diameter drills within a PCB and multiple conductive material filled cores having a connecting inner layer trace (target layer) and adjacent conductive material filled prepreg for forming a via without a stub.  FIGS. 18A-18H  illustrate different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg between conductive material filled cores; and having a connecting inner layer trace (target layer) for forming a via without a stub, according to one aspect. 
         [0096]    In the method, three different drill sizes may be utilized to drill through an upper surface  1800   a  and lower surface  1800   b  of a PCB  1800  passing through embedded conductive material in the PCB  1800 . First, a first hole  1802  is drilled in the core  1804  and prepreg  1806  having a diameter of d 1  and filled with conductive material  1808 , such as a conductive paste or plated copper  1808 . Next, a second hole  1810 , having a diameter of d 2 , is drilled after lamination, until reaching the embedded conductive material  1808 . Next, a third hole  1812 , having a diameter of d 3 , is drilled through the second hole  1810 , where d 3  is smaller than d 2  and d 1 . The second and third holes  1810  and  1812  are then plated over with conductive material  1814 , such as copper. Next, plated conductive material is removed from the surface of the third hole  1812  by drilling down through the second hole  1810  forming a fourth hole  1816  having a diameter of d 4 , where d 4  is larger than d 3  and smaller than d 1  and d 2 . (See  FIGS. 18E-18H ) 
         [0097]      FIGS. 19A-19G  the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg and solid core as well as having a connecting inner layer trace (target layer) for forming a via hole without a stub. 
         [0098]    In the method, four different drill sizes are utilized to drill through an upper surface and lower surface of a PCB  1900  passing through embedded conductive material in the PCB  1900 . A first hole  1902 , having a diameter d 1 , is drilled in the pregreg  1906  and filled with conductive material  1908  such as conductive paste. 
         [0099]    Next, a second hole  1910 , having a diameter of d 2 , is drilled after lamination until reaching the top of the embedded conductive material  1908 . Next, a third hole  1912  having a diameter d 3  is drilled, where d 3  is smaller than d 1  and d 2 . Next, the second hole  1910  and the third hole  1912  are plated over by a conductive material  1914  such as copper. The third hole  1912  is then is back drilled to remove the conductive material from the inner surface of the third hole  1912  and form a fourth hole having a diameter of d 4 , where d 4  is larger than d 3  and smaller than d 1 . (See  FIGS. 19E-19G ) 
         [0100]      FIGS. 20A-20H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB having a conductive material filled prepreg  2004  between a conductive material filled core  2006  and a solid core  2008 ; and a connecting inner layer trace (target layer) for forming a via without a stub. 
         [0101]    In the method, five different drill sizes are utilized to drill through an upper surface and lower surface of a PCB  2000  passing through embedded conductive material in the PCB  2000 . A first hole  2002  and a second hole  2003  are drilled in the pregreg  2004  and the core  2006  having a diameter of d 1  and d 2 , respectively ( FIG. 20A ), filled with conductive material  2010 , such as conductive paste or plated copper, and an inner layer circuit is formed on the core  2006  ( FIG. 20B ). Then the cores and prepreg are laminated aligning stacked conductive paste in the core and prepreg to form PCB  2000 . ( FIG. 20D ) 
         [0102]    Next, a third hole  2012 , having a diameter of d 3 , is drilled after lamination until reaching the top of the embedded conductive material  2010 . Next, a fourth hole  2014  having a diameter d 4  is drilled, where d 4  is smaller than d 3 . Next, the third hole  2012  and the fourth hole  2014  are plated over by a conductive material  2016  such as copper. Next, the fourth hole  2014  is back drilled to remove the conductive material and form a fifth hole having a diameter of d 5 , where d 5  is larger than d 4  and smaller than d 3 . (See  FIGS. 20E-20H ) Alternatively, the fifth drill may be applied after conductive seeding prior to electrolytic plating. 
         [0103]    In another aspect, the present disclosure provides a method of using different diameter drills within a PCB and a conductive material filled core having a connecting inner layer trace (target layer) and adjacent conductive material filled prepregs for forming a via without a stub.  FIGS. 21A-21H  illustrate the different fabrication stages for forming a plated through hole (or via) using different diameter drills within a PCB and multiple conductive material filled cores having a connecting inner layer trace (target layer) and adjacent conductive material filled prepregs for forming a via without a stub. 
         [0104]    In the method, six different drill sizes are utilized to drill through an upper surface and lower surface of a PCB  2100  passing through embedded conductive material in the PCB  2100 . A hole  2102   b , having a diameter d 2 , is drilled in the core  2104   b  and holes  2102   a  and  2102   c , having a diameter d 1  and d 3  respectively, are drilled in the first and second prepregs  2104   a  and  2104   c , and filled with conductive material  2106 , such as a conductive paste or plated copper. The core  2104   b  is formed on a circuit on the surface  2108 . The materials are laminated to make a PCB  2100  aligning conductive material filled holes. Next, a fourth hole  2110 , having a diameter d 4 , is drilled after lamination, until reaching the embedded conductive material  2106 . Next, a fifth hole  2112 , having a diameter d 5 , is drilled through the PCB  2100  and the embedded conductive material  2108 , where the fourth hole  2110  and the fifth hole  2112  are plated over with conductive material  2114 , such as copper. Next, the fifth hole  2112  is back drilled to remove the conductive material from the fifth hole  2112 , while maintaining the conductive material in the fourth hole  2110 , and forming a sixth hole  2116  having a diameter of d 6 , where d 6  is larger than d 5  and smaller than d 1 , d 2 , d 3  and d 4 . (See  FIGS. 21E-21H ) Alternatively, back drilling may proceed after conductive seeding for electrolytic plating, such as electroless plating catalyst or electroless copper, prior to electrolytic plating. 
       Formation of Plated Through Hole with High Aspect Ratio 
       [0105]    Referring to  FIGS. 22A-22G , the different fabrication stages for forming a plated through hole (or via)  2220  having a high aspect ratio in a printed circuit board (PCB)  2201  are illustrated. 
         [0106]    As described below, a three (3) drill step process is used to form the plated through hole. In the first step, a first hole  2206  is drilled, from the lower surface  2201   b  of the PCB, using a first drill  2208  having a first diameter d 1 . The first hole  2206  is drilled approximately half way through the PCB  2201 . (See  FIGS. 22A and 22B ) Next, the second drill  2209 , having a second diameter d 2 , is used to drill, from the upper surface  2201   a  of the PCB, down to a top of the first hole  2206  forming a second hole  2210 . An offset  2212  may be created at the end of the second hole  2210  and the beginning of the first hole  2206 . The first hole  2206  and the second hole  2210  together form a hole extending through the entire vertical length of the PCB. (See  FIG. 22D ) 
         [0107]    Next, a third drill  2214  is used to drill from the lower surface  2201   b  upwards (or alternatively the upper surface  2201   a  downwards) through the first hole  2206  and the second hole  2210 . The third drill  2214  has a third diameter d 3  which is larger than the first diameter d 1  of the first drill  2208  and the second diameter d 2  of the second drill  2208 . As a result, a smooth hole  2216  is formed that extends through the entire vertical length of the PCB  2201 . (See  FIGS. 22E-22F ) The smooth hole  2216  is then plated  2218  with a conductive material forming a plated through hole  2220 . (See  FIG. 22G ) 
         [0108]      FIG. 23  illustrates a method for forming a plated through hole, having a high aspect ratio, in a PCB, according to one aspect. In the method, a first hole is drilled, from a first surface of the PCB to a predetermined depth using a first drill having a first diameter  2302 . For example, the predetermined depth is half way through the PCB. Next, a second hole is drilled, using a second drill having a second diameter, from a second surface of the PCB, the second surface opposite the first surface, to an upper end of the first hole. These holes may create an offset between a lower end of the second hole and the upper end of the first hole due to drill process accuracy  2304 . Next, a third hole is drilled, using a drill having a third diameter which is larger than both the first and the second drill diameter, through the first and second holes smoothing the offset between the first and second holes  2306 . The third hole is then plated with a conductive material forming a plated through hole  2308 . 
         [0109]    In another aspect, the present disclosure provides a method of forming a plated through hole, having a high aspect ratio, in a PCB. (See  FIGS. 24A-24D ) In the method, two different drill sizes may be utilized to drill through an upper surface  2400 a and lower surface  2400 b of a PCB. First, a first hole  2402  is drilled approximately half way through the PCB  2400  using a first drill  2404 . Next, the first drill  2404  is used to drill a second hole  2406  from the bottom surface  2400 b of the PCB  2400  to the bottom of the first hole  2402 . An offset  2408  is created between the first hole  2402  and the second hole  2406  in the PCB  2400 . A second drill  2408 , where the second drill  2408  is larger than the first drill  2404  (i.e. has a larger diameter), is then used to drill a third hole  2410  through both the first hole  2402  and the second hole  2406 . As the second drill  2408  has a diameter larger than the diameter of the small drill  2404 , the first and second holes  2402  and  2406  work as a guide and are smoothed by the large diameter drilling. A conductive material  2412  is then plated on the inner surface of the hole  2410  forming a plated through hole which extends the vertical length of the PCB. 
         [0110]    While certain exemplary aspects have been described and shown in the accompanying drawings, it is to be understood that such aspects are merely illustrative of and not restrictive on the broad invention, and that this invention not be limited to the specific constructions and arrangements shown and described, since various other modifications may occur to those ordinarily skilled in the art.