Abstract:
A method and apparatus for shaping signals transmitted via plated through holes (PTHs) utilizes standard back-drilling techniques to reduce the resonant stub lengths of ground PTHs in the vicinity of a back-drilled signal PTH.

Description:
BACKGROUND OF THE INVENTION 
   The system design aspect of recent years has driven the requirements for special tools and practices to ensure high speed signaling quality, especially for backplanes and connector via arrays. 
   The speeds of signals used in the industry increases at about an average rate of twice every two years. As a result the rise time of signals used on backplanes and other printed circuit boards (PCBs) decreases and the bandwidth required to deliver those signals from point to point increases (doubles every two years). Transport data rates of 3.125 Gigabits per second (Gbps) are now commonplace in board-to-board applications. As data rates increase to 5, 6.25, or 10 Gbps each part of the channel must be examined to increase performance. 
   A channel includes plated through holes (PTHs), also called vias, that transport signals into interior layers of a multi-layer PCB as depicted in  FIG. 1 . PTHs are common to many device packages such a Ball Grid Arrays (BGAs) and other connector types. Typically, in a backplane system a signal path between a transmitter and receiver includes several vias or PTHs. 
   PTHs are fabricated by drilling a hole through a multi-layer PCB. As is known in the art, a multi-layer PCB includes conductive traces separated by dielectric layers. The hole is plated with a conductor, such as copper, and a pad is formed to connect the PTH to a particular one of the conductive traces. As depicted in  FIG. 1 , a PTH  10  may be utilized to conduct a signal from a capture pad  12  mounted on the surface  14  of the PCB to an internal trace  16 . In this case, the PTH has a pad on the surface for connecting with the capture pad and another pad connected to the selected internal trace. The PTH is insulated from all other traces. 
   At high frequencies the PTH joining a surface pad to an interior trace will affect signal shaping. Depending on the frequency of the signal and the dimensions of the PTH, signal energy may be reflected from the PTH or converted into radiation thus causing loss of signal power and other undesirable side effects. 
   It is known that the frequencies where the PTH acts as a filter are determined by the unused portion of the hole, referred to as the resonant stub. In  FIG. 1 , the portion of the PTH that extends beyond the selected trace layer is the resonant stub  18 . One technique for controlling the effects of the stub is to alter its size by a technique known as “back-drilling”, where plating is removed from the unused portion of the PTH by drilling from the back side of the PCB as depicted in  FIG. 2 . Back-drilling necessarily entails a tradeoff between manufacturing costs and electrical performance. Its effectiveness is limited by drilling depth accuracy and the increased cost of multi-depth drilling. 
   However, these simple back-drilling techniques (used to reduce the stub effect) in some cases are not adequate to guarantee high quality signaling at the speed of 2 Gbps and above. 
   Other known techniques exist that utilize more exotic technology, such as embedded passive or active filters in the PCB, to reduce reflections and shape signals. However, those techniques are expensive and, in most cases, not useful in mass production of PCBs. 
   The challenges in the field of high-frequency transmission continue to increase with demands for more and better techniques having greater simplicity and lower cost. Therefore, a need has arisen for a new system and method for controlling reflection and signal shaping caused by PTHs. 
   BRIEF SUMMARY OF THE INVENTION 
   In one embodiment of the invention, standard back-drilling technology is utilized to remove resonant stubs of ground PTHs in the vicinity of a back-drilled signal PTH to shape a signal and reduce reflections. 
   In another embodiment of the invention, the stub lengths of ground PTHs are varied to control signal shaping at selected signal frequencies. 
   Other features and advantages of the invention will be apparent in view of the following detailed description and appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective, cut-away view of a plated through hole; 
       FIG. 2  comprises cross-sectional views depicting the back-drilling process; 
       FIG. 3  is top view of a layout of signal and ground PTHs; 
       FIG. 4  is a side view of the layout of  FIG. 3 ; 
       FIG. 5  is a perspective, cut-away view of an embodiment of the invention; 
       FIG. 6  is a graph illustrating the operation of an embodiment of the invention; and 
       FIG. 7  is a flow chart depicting steps performed by an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to various embodiments of the invention. Examples of these embodiments are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it will be understood that it is not intended to limit the invention to any embodiment. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. However, the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
     FIGS. 3 and 4  depict a layout of PTHs on a printed circuit board. As is known in the art, signal PTHs are usually placed between ground PTHs to reduce cross-talk between signals. In  FIG. 3 , signal PTH  30  is placed between first, second, third, and fourth ground PTHs  32 ( 1 )- 32 ( 4 ).  FIG. 4  is a side views depicting a signal PTH  30  that has been back-drilled to reduce the size of the resonant stub and that is surrounded by the four ground PTHs  32 ( 1 )- 32 ( 4 ). 
   The inventors have discovered that the signal quality of a channel and the ability to transfer high frequency signals through a variety of board thicknesses can be improved by applying standard back-drilling techniques to reduce the resonant stub length of the ground PTHs surrounding a signal PTH. 
     FIG. 5  depicts a preferred embodiment of the invention where ground PTHs  32 ( 1 )- 32 ( 4 ) have been back-drilled so the resonant stubs of the ground PTHs have been reduced in size. 
     FIG. 6  is a graph depicting the attenuation of a signal at various frequencies. The dotted line depicts a significant increase in signal attenuation at 15 Ghz. As depicted by the dashed line, this signal attenuation is much less pronounced after the ground PTHs have been back-drilled. The effect of the ground PTH back-drilling as shown in the graph makes the channel much more “flat”, i.e. there is much less resonance and reflection on the channel. This has a direct effect on the signal quality at the output of that channel/via structure. 
   In the embodiment described above standard back-drilling techniques are utilized. The signal and ground PTHs can be back-drilled during the same fabrication without adding significant cost or complexity to manufacturing a PCB. 
   In a preferred embodiment, the ground PTHs in a backplane are back-drilled, however the technique is useful in any board used to form high-frequency signal transmission channels. 
   In another embodiment of the invention, the lengths of the resonant stubs of the signal and ground PTHs can be varied to shape the transmitted signal according to the requirements of the channel. 
   In this embodiment of the invention, signal PTH back-drilling combined with the GND PTH back-drilling creates a filter effect on the signal. As depicted in the flow chart of  FIG. 7 , the filtering formula is changed by adjusting the depth of the back-drilling of the signal PTH and back-drilling the ground PTHs to the same or different depths. 
   Using modeling tools (like HFSS manufactured by Ansoft) the transformation formula of the channel is extracted for every back-drilling depth and the required signal filtering or shaping can be set. Thus, the output signal can be controlled, filtered and shaped according to the transformation formula of the PTH structure and can be adjusted by changing the back-drilling depths. 
   The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of skill in the art. For example, the diagrams depict a signal PTH surrounded by four ground PTHs, however, the invention is not limited to any number or configuration of PTHs. Further, modeling programs other than HFSS can be utilized as is understood by persons of skill in the art. Accordingly, it is not intended to limit the invention except as provided by the appended claims.