Abstract:
The present invention is directed to a transmission line assembly and method of propagating signals therethrough that features forming transmission lines of the assembly to provide desired filtering properties. To that end, the assembly includes a plurality of spaced-apart transmission lines placing first and second sets of active circuits in electrical communication, with a subset of the plurality of spaced apart transmission lines having dimensions to filter unwanted characteristics of signals, propagating between the first and second sets and inductively coupled between one or more of the plurality of spaced-apart transmission lines. The method performs the function of the assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/970,220, filed Sep. 5, 2007, and entitled “SIMULTANEOUS SWITCHING NOISE FILTER ARCHITECTURE AND METHOD.” This provisional application is herein incorporated by reference. 
    
    
     BACKGROUND 
     The present invention relates to integrated circuit systems and more particularly to filtering techniques to reduce simultaneous switching noise between transmission lines disposed in printed circuit boards (PCBs) and placing integrated circuits in signal communication, referred to as integrated circuit assemblies (ICAs). 
     During normal operations of the ICAs, close proximity of transmission lines cause inductively coupling of signals between adjacent transmission lines in the presence of a time varying current in one of the same. Inductively coupling of signals, in this manner, is typically referred to simultaneous switching noise (SSN). SSN may interfere with operation of the integrated circuit resulting in faulty operation of the same. As a result, there have been several attempts to reduce switching noise. 
     An existing technique to reduce SSN employs multiple low-inductance bypass, or decoupling, capacitors. Decoupling capacitors filter noise by “short circuiting” high frequency components of a noise signal and are often connected between each power plane and adjacent ground plane. However, the inclusion of additional components, such as capacitors, results in increased cost of production of ICAs. 
     A need exist, therefore, to provide improved ICAs manufacturing techniques. 
     SUMMARY 
     It should be appreciated that the present invention can be implemented in numerous ways, such as a process and a package. Several inventive embodiments of the present invention are described below. 
     The present invention is directed to an integrated circuit assembly and method of propagating signals therethrough that features forming transmission lines of the assembly to provide desired filtering characteristics. To that end, the integrated circuit assembly includes first and second sets of active circuits and a plurality of spaced-apart transmission lines placing the first and second set of active circuits in electrical communication. A subset of the plurality of spaced apart transmission lines have dimensions to filter unwanted characteristics of signals propagating between the first and second sets and inductively coupled to one or more of the plurality of spaced-apart transmission lines. An integrated circuit having spaced apart transmission lines with dimensions to filter unwanted characteristics of signals propagating between the first and second sets is provided in another aspect of the invention. 
     These and other aspects of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be best understood by reference to the following description taken in conjunction with the accompanying figures, in which like parts may be referred with like numerals. 
         FIG. 1  is a simplified cross-sectional view of an integrated circuit system in accordance with one embodiment of the present invention; 
         FIG. 2  is a schematic of a portion of the integrated circuit system, shown in  FIG. 1 , coupled to loads in accordance with the present invention; 
         FIG. 3  is simplified graph showing signal propagation along one of the channels shown in  FIG. 1 ; 
         FIG. 4  is a simplified electrical schematic showing a basic filter configuration in accordance with the present invention; 
         FIG. 5  is a simplified schematic showing the implementation of the electrical functions shown in the electrical schematic of  FIG. 4 ; 
         FIG. 6  is a graph comparing the signal noise generated by inductively coupling between transmission lines shown in  FIG. 2 ; 
         FIG. 7  is a graph comparing the signal output from one of the transmission lines mentioned in  FIG. 6 ; 
         FIG. 8  is a simplified plan view showing the dimension of the transmission lines shown in  FIG. 1  to implement the filter shown in  FIG. 4 , in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an integrated circuit system  10  is shown as including a substrate  12 , typically a printed circuit board (PCB) having a plurality of vias  14  and a plurality of conductive transmission lines  18 ,  19 ,  20  and  21  disposed upon one side thereof in electrical communication with one or more of vias  14 . A plurality of contact pads  16  is disposed on a side of substrate  12  that is opposite to the side upon which conductive transmission lines  18 ,  19 ,  20  and  21  are disposed and in electrical communication with one or more of vias  14 . Vias  14  place conductive transmission lines  18 ,  19 ,  20  and  21  in electrical communication with different subsets of output contact pads  16 . Integrated circuit  22  includes a plurality of bonding pads  24  and is mechanically and electrically coupled to substrate  12  by solder bumps  26  disposed between bonding pads  24  and conductive transmission lines  18 ,  19 ,  20  and  21 , using techniques well known in the art, discussed further below. Signals from integrated circuit  22  are transmitted outside of integrated circuit package  10  by solder bumps  28  that are attached to and in electrical communication with contact pads  16 . Solder bumps  28  are also used to place other circuits, such as integrated circuit  30 , in electrical communication with integrated circuit  22 . 
     Referring to both  FIGS. 1 and 2 , typically integrated circuit  22  includes a plurality of active circuits  32  defining a first set  34 . Integrated circuit  30  includes a plurality of active circuits  36 , defining a second set  38 . First and second sets  34  and  38  are in electrical communication via a set  40  of transmission lines  18 ,  19 ,  20  and  21 . 
     Referring to both  FIGS. 2 and 3 , signals, such as signal  42  propagate between first and second seconds  34  and  38  over set  40  of conductive transmission lines  18 ,  19 ,  20  and  21 . As is well known, the physical proximity of adjacent conductive transmission lines  18 ,  19 ,  20  and  21  may attribute to cross-coupling of signals propagating between first and second sets  34  and  38 . The cross-coupling results from a change in current flow through one of conductive transmission lines  18 ,  19 ,  20  and  21  that occurs as a result of a transition of signal  42  from a logical zero “0” voltage level  44  to a logical “1” voltage level  46 . This produces a magnetic field, B, that is shared between one or more adjacent conductive transmission lines  18 ,  19 ,  20  and  21  inductively coupling a signal by inducing current flow, referred to as cross-talk or simultaneous switching noise (SSN). SSN presents as an inductively coupled signal  48  on the conductive transmission lines  18 ,  19 , and  21  in which the induced current is present. As the magnitude of inductively coupled signal  48  approaches voltage level  44 , active circuit  36  receiving the same may incorrectly identify the same as signal  42 . This may be deleterious to the operation of active circuit  36 . 
     Referring to  FIGS. 2 ,  3  and  4 , one manner in which to attenuate the characteristics of inductively coupled signal  48  is to provide an RLC filter  50  that includes inductive components  51  and  52 , capacitive components  53  and  54  and resistive components  55  and  56 . Inductive components  51  and  52  are connected in series and capacitive components  53  and  54  are connected in parallel between ground and opposed side of inductive component  52 . Resistive component  55  is coupled in parallel with capacitive component  53  between ground and opposed sides of inductive component  51 , and resistive component  56  is coupled in parallel with capacitive component  54  between ground and a common side of inductive component  52 . 
     To avoid the increased cost associated with including inductive components  51  and  52  and capacitive components  53  and  54  to assembly  10 , filter  50  is implemented in assembly  10  by establishing dimensions of conductive transmission lines  18 ,  19 ,  20  and  21  to provide desired filtering properties. To that end, each of conductive transmission lines  18 ,  19 ,  20  and  21  includes filter segments  60 ,  61 ,  62  and  63  that provide the aforementioned filter properties to form a transmission line filter  150 , shown in  FIG. 5 . Transmission line filter  150  includes resistive components  55  and  56  that correspond to the resistance presented by active circuits  32  and  36 , respectively, at opposed ends of conductive transmission lines  18 ,  19 ,  20  and  21 . Inductive components  51  and  52 , as well as, capacitive components  53  and  54  have been replaced appropriate dimensions of material from which filter segments  60 - 63  of conductive transmission lines  18 ,  19 ,  20  and  21  are formed, shown as  64 ,  65 ,  66  and  67 . 
     The dimensions of segments  60 - 63  of conductive transmission lines  18 ,  19 ,  20  and  21  are configured to provide desired filtering properties. The filtering properties are a function of a coupling component [M], which represents inductively coupling characteristics of adjacent conductive transmission lines  18 ,  19 ,  20  and  21  in the presence of a time varying current di/dt associated with signal  42  on one of conductive transmission lines  18 ,  19 ,  20  and  21 . Specifically, a magnitude of inductive coupled signal  48  V on one of conductive transmission lines  18 ,  19 ,  20  and  21  may be expressed as follows:
 
 V   m   =[M   n   ]di/dt  
 
where V m  is the transmission line upon which signal  48  is present and M n  is the coupling component between the conductive transmission lines  18 ,  19 ,  20  and  21  upon which signal  42  and the conductive transmission lines  18 ,  19 ,  20  and  21  upon which inductively coupled signal  48  is present. Time varying current di/dt is the change of current present when signal  42  alternates between a logic “0” voltage level  44  and a logic “1” voltage level  46  and vice-versa.
 
     Referring to  FIGS. 2 ,  6  and  7 , it was determined that many different filtering properties may be employed. For example, dimensions of filtering segments  60 - 63  may be established to provide a Chebyshev filter, a Bessel filter and the like. As shown, a magnitude of an unfiltered inductive coupled signal is shown by curve  70  to be in excess of 200 millivolts. Implementation of Bessel filtering properties in segments  60 - 63  results in a reduction of noise to a little greater than less than 100 millivolts, shown by curve  72 . Implementation of Chebyshev filtering properties in segments  60 - 63  results in a further reduction of noise to less than 100 millivolts. However, Chebyshev filtering properties results in a distortion  76 . This results from the sharp cutoff frequency response of Chebyshev filters. Distortion  76  feeds back to the transmission line and therefore, the signal  42  propagating thereon that produces the SSN represented by curve  74 , as shown by signal  80 , with a curve  82  representing output from the same transmission line having Bessel filtering properties. As a result, it is desired to provide segments  60 - 62  with dimensions to provide Bessel filtering properties. For example, it is desired that filtering properties attenuate a bandwidth of inductively coupled signal  48 , as measured in the frequency domain, while maintaining the magnitude to be below a threshold voltage associated with the active circuits  36 . 
     In the present example, the threshold voltage is defined as the voltage at which point field effect transistors (not shown) within active circuits  36  begin to operate, e.g., “turn-on”. It should be noted that the filtering properties are, therefore, determined based upon the active circuits  36 , the parameters of signal  42 , the materials from which segments are formed and the parasitic characteristics of coupling active circuits  32  and  36  to set  40  of conductive transmission lines  18 ,  19 ,  20  and  21 . 
     Referring to  FIG. 8 , to that end, one example of segments  60 - 63  includes providing a copper trace  90  have a length of approximately 762 micrometers, a width of approximately 40 micrometers and a thickness of approximately 17 micrometers. Extending from trace  90  is a trace  92  with a length of approximately 18 micrometers, a width of approximately 20 micrometers and a thickness of approximately 17 micrometers. Extending from a junction  94  of traces  90  and  92  is a triangular portion  96 , the sides of which have a common length, e.g., 50 micrometers. Disposed proximate to a terminus of trace  92 , positioned opposite to junction  94  is a second triangular portion  98 , the sides of which have a common length, e.g., 100 micrometers. In one embodiment, the angle of the sides of the triangular portions  96  and  98  with the bottom of traces  90  and  92  is about 60 degrees. It is possible to form the features  90 ,  92 ,  94 ,  96  and  98  to have larger sizes, e.g., when formed on a PCB; however, it is desired that the aforementioned ratios of dimensions be substantially maintained. Additionally, it is possible to fabricate vias  14  to provide the function provided by features  92 ,  94 ,  96  and  98 , in lieu of conductive transmission lines  18 ,  19 ,  20  and  21  or, alternatively, in conjunction therewith. Thus, the traces can be implemented in a horizontal and vertical direction. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Accordingly, the present embodiments described above are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may defined by the appended claims, including full scope of equivalents thereof.