Abstract:
A printed circuit board assembly characterized by at least two decoupling capacitors for decoupling transient currents resulting from, for example, logic transitions in high-speed digital circuitry. The decoupling capacitors are physically arranged, and electrically connected between a power plane and a ground plane, so that transient currents flow in respectively opposite directions through the capacitors, thereby maximizing the capacitors&#39; mutual inductance, and thus minimizing the electromagnetic interference generated by the capacitors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the design and fabrication of electronic equipment and, more particularly, to the design of a printed circuit board (PCB) that increases electromagnetic coupling between discrete decoupling capacitors mounted on the surface(s) of such PCB. 
     2. Description of the Related Art 
     A significant consideration in the design and fabrication of compact (and therefore densely assembled), high-speed digital equipment is the need to minimize the effects of ringing, crosstalk, radiated noise and other forms of electromagnetic interference (EMI). However, design approaches seeking to minimize EMI effects are generally susceptible to straight forward circuit analysis. In fact, although entire textbooks have been devoted to techniques for combating EMI, the subject continues to be viewed as “black magic”. See, for example, Howard Graham, “High-Speed Digital Design,” Prentice Hall PTR, Saddle River, N.J. (1993). 
     High-speed digital circuits and systems frequently draw large transient currents during short intervals, when, for example, logic circuits and devices change state. Often logic transitions take place with brief rise and fall times, under the control of increasingly high-frequency clock signals. Because realizable voltage sources for digital circuitry are characterized by series resistances and inductances, decoupling capacitors are commonly relied on to supply transient current requirements during transition intervals. The coupling capacitors are typically electrically connected between a voltage supply and ground and serve to mitigate the effects of the nonzero voltage supply source impedance. The decoupling capacitors therefore, tend to maintain the output of the voltage supply by providing a significant portion of the transient current. 
     However, the ability of commercially available capacitors to supply current at high frequencies is limited by the parasitic lead inductance that is characteristic of such capacitors. In addition to the inductance associated with capacitor leads, the finite inductance of each via that may be used, for example, to attach a power supply plane to a ground plane introduces a small, but measurable inductance. The magnitude of this inductance is approximately:        L   =     5.08                   h        [       ln        (       4      h     d     )       +   1     ]                                
     where 
     L=inductance of via, nH, 
     h=length of via, inches 
     d=diameter of via, inches 
     Parasitic inductance is unavoidable because current flowing in a capacitor will create lines of flux. The effect of parasitic lead inductance is aggravated by the additional inductance that is inherent to the physical layout of electronic circuitry. This additional inductive component arises both from the conductive traces imparted to the PCB, as well as from the physical proximity of discrete components, both active and passive. 
     EMI may be encountered in PCBs in at least two modes. As suggested above, differential mode (DM) EMI results from currents that flow in a loop through various components or circuits on the PCB. DM EMI can be reduced either by reducing the current that flows in the loop, or by reducing the area subtended by the loop. EMI is also encountered as a result of voltage drops related to induced impedance. The form of EMI is referred to as common mode (CM) EMI because it may be transmitted across an entire circuit. CM EMI may be reduced by decreasing the rate of change of current (dI/dt) that causes the CM EMI, or by decreasing the inductance in the current path. 
     Decoupling capacitors are, as with any component of high-speed digital circuit, susceptible to both DM and CM EMI phenomena. A prevalent approach to reducing the effects of parasitic inductance associated with decoupling capacitors is simply to connect a number of decoupling capacitors in parallel between the voltage supply and GND. When the decoupling capacitors are connected in parallel, the effective parasitic inductance is reduced, approximately by a factor equal to the number of capacitors that are connected in the parallel configuration. It is not surprising, then, that a number of commercially available discrete capacitors, and capacitor arrays, are designed in this manner to minimize the parasitic inductance of capacitors. Specifically, AVX Corporation, Myrtle Beach, S.C., manufacturers and distributes a product line of capacitor configurations under various product designations, such as DCAP, Low Inductance Capacitor Array (LICA), the Power Plane Decoupling Capacitor, the Interdigital Capacitor and the Reverse Terminal Capacitor. Capacitor arrays such as the above and others are recognized as affording improved decoupling, with reduced parasitic inductance. However, such arrays typically command a price premium when compared to the standard surface mount technology (SMT) decoupling capacitor. 
     Accordingly, what is desired is an economical approach to the arrangement of multiple decoupling capacitors on a printed circuit board. The objective is to reduce parasitic inductance as well as EMI effects. Furthermore, it is another object of the invention to further reduce EMI by limiting the mutual inductance exhibited between capacitor bodies that are situated in proximity on a PCB. Reduction of mutual induction will also result in corresponding reductions in CM EMI. An additional desired result is the reduction in DM EMI by constraining the area of the current loop circumscribed by the decoupling capacitors. Finally, it is preferred that the above results be achieved with commonly available, discrete SMT decoupling capacitors, thereby avoiding premium prices that accompany exotic capacitor arrays. 
     SUMMARY OF THE INVENTION 
     The above and other objects, advantages and capabilities are achieved in one aspect of the invention by circuit assembly that comprises a printed circuit board (PCB) that is characterized by a first exterior surface, a first internal conductive layer, and a second internal conductive layer. First and second discrete capacitive elements, each having conductive terminals disposed at opposing extremities of the associated capacitive element, are laterally juxtaposed on the first exterior surface of the PCB so that the first conductive terminal of the first capacitive element is disposed adjacent to the first conductive terminal of the second capacitive element and is disposed remotely from the second conductive terminal of the second capacitive element. A first conductor couples the first conductive terminal of the first capacitive element to the first internal conductive layer; a second conductor couples the second conductive terminal of the first capacitive element to the second internal conductive layer; a third conductor couples the first conductive terminal of the second capacitive element to the second conductive layer; and a fourth conductor couples the second conductive terminal of the second capacitive element to the first internal conductive layer. 
     Another aspect of the invention contemplates a printed circuit board (PCB) characterized by a first exterior surface, a first internal conductive layer, and a second internal conductive layer. First and second discrete capacitive elements, each having conductive terminals disposed at opposing extremities of the associated capacitive element, are mutually juxtaposed on the first exterior surface of the PCB so that the first conductive terminal of the first capacitive element is disposed adjacent to the first conductive terminal of the second capacitive element and remotely from the second conductive terminal of the second capacitive element. A conductor array couples two of the conductive terminals to the first internal conductive layer and couples two of the conductive terminals to the second internal conductive layer. 
     Another additional manifestation of the invention is represented by a circuit assembly that comprises a printed circuit board (PCB) characterized by a first exterior surface, a second exterior surface, a first internal conductive layer, and a second internal conductive layer. First and second discrete capacitive elements each have respective first and second conductive terminals disposed at opposing extremities of the associated capacitive element The first capacitive element is positioned on the first exterior surface, and the second capacitive element is positioned on the second exterior surface, substantially orthogonally aligned with the first capacitive element. A first conductor couples the first conductive terminal of the first capacitive element to the first internal conductive layer; a second conductor couples the second conductive terminal of the first capacitive element to the second internal conductive layer; a third conductor couples the first conductive terminal of the second capacitive element to the second internal conductive layer; and a fourth conductor couples the second conductive terminal of the second capacitive element to the first internal conductive layer. 
     Yet another ramification of the invention is embodied in a printed circuit board adapted for the arrangement of at least two capacitive elements, each of the capacitive elements having respective first and second conductive terminals. The PCB comprises an exterior surface for supporting the capacitive elements, a first internal conductive layer, a second internal conductive layer, and a conductive array for selectively coupling each of the conductive terminals of the capacitive elements to either the first internal conductive layer or the second internal conductive layer and for reducing electromagnetic coupling between the capacitive elements. 
     Similarly, the invention resides in a printed circuit board (PCB) adapted for the arrangement of at least a first capacitive element and a second capacitive element, each of which capacitive elements has respective first and second conductive terminals. The PCB comprises a first exterior surface for supporting the first capacitive element, a second exterior surface for supporting the second capacitive element, a first internal conductive layer, a second internal conductive layer, and a conductive array for selectively coupling each of the conductive terminals of the respective capacitive elements to either the first internal conductive layer or the second internal conductive layer and for reducing the electromagnetic coupling between the capacitive elements. 
     The invention may also be exploited in various types of electronic equipment, including, but not limited to, computer systems, which equipment includes printed circuit assemblies comprising multilayer printed circuit boards (PCBs). The PCBs are characterized by a first and second exterior surface, and by first and second internal conductive layers. The printed circuit assembly also comprises first and second discrete capacitors, juxtaposed on the PCB in a manner that tends to minimize the capacitor&#39;s mutual inductance. A first conductor couples the first conductive terminal of the first capacitive element to the first internal conductive layer; a second conductor couples the second conductive terminal of the first capacitive element to the second internal conductive layer; a third conductor couples the first conductive terminal of the second capacitive element to the second conductive layer; and a fourth conductor couples the second conductive terminal of the second capacitive element to the first internal conductive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art, with reference to the Drawings described below and attached hereto, in the several Figures of which like reference numerals identify indentual elements and where: 
     FIG. 1A is top view of a pair of laterally juxtaposed capacitors on a PCB. 
     FIG. 1B is an end view, illustrating cancellation of flux lines as a result of the capacitor arrangement of FIG.  1 A. 
     FIGS. 1C and 1D are cross-sectional views, illustrating in detail the manner in which the laterally juxtaposed capacitors are electrically interconnected. 
     FIG. 2 is a cross-sectional view of an embodiment of the invention in which a capacitor pair is orthogonally juxtaposed about opposite surfaces of a PCB. 
     FIG. 3 is a cross-sectional view of an embodiment of the invention in which a capacitor pair is axially aligned on a surface of a PCB. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a thorough understanding of the subject invention, reference is made to the following Description, which includes the appended Claims, in connection with the above-described Drawings. 
     Refer now to FIG. 1 (including FIGS. 1A,  1 B,  1 C and  1 D), which depicts an embodiment of the invention according to which a pair of decoupling capacitors, C 1  and C 2 , are mounted on an upper exterior surface  31 , a printed circuit board  3 . In a particular embodiment of the invention, PCB is a multilayer board, the various layers of which may be more readily discerned in FIG. 1C, which is a cross-sectional view taken along the plane C 1 —C 1 , and FIG. 1D, which is a cross-sectional view taken along the plane D 1 —D 1 . 
     FIG. 1 depicts an embodiment in which the decoupling capacitors are laterally juxtaposed on PCB  1 . As may be seen in FIGS. 1C and 1D, each of the coupling capacitors comprises a respective dielectric body portion ( 11 ,  21 ), first conductive terminal ( 12 ,  22 ) and second conductive terminal ( 13 ,  23 ). If each of the decoupling capacitors assumes the form of a rectangular parallelepiped, then respective axes may be defined as extending through the center of the respective body portions of the capacitors and between the centers of the respective first and second conductive terminals. With the capacitor axes so defined, then lateral juxtaposition, as used above, signifies that the axis of C 1  extends in a manner parallel to the axis of C 2 , and that both axes reside in an imaginary plane (not shown) that is substantially parallel to the first exterior surface of the PCB  3 . 
     The essence of the inventive embodiment depicted in FIG. 1 is that the decoupling capacitors C 1  and C 2  are electrically interconnected by a conductive array  4  that comprises conductive pads  41 ,  42 ,  43 , and  44 . The conductive array also includes vias  45 ,  46 ,  47 , and  48 . In a manner well understood by those skilled in the art, pads  41 ,  42 ,  43 , and  44  are etched onto surface  31  of PCB. C 1  is mounted on PCB by connecting, as by soldering, conductive terminal  12  to pad  41  and conductive terminal  13  to pad  42 . Similarly, conductive terminal  21  of C 2  is connected to pad  43 , and conductive terminal  22  is connected to pad  44 . 
     As may be seen in FIG. 1C, the first conductive terminal  12  of capacitor C 1  is electrically coupled through a conductor comprising pad  41  and via  45  to a first internal conductive layer  32 . Via  45  extends from surface  31  of PCB  3  to conductive layer  32 . In the present embodiment of the invention, conductive layer  32  is a power plane that is coupled to a voltage supply, V+. Similarly, the second conductive terminal  13  of C 1  is electrically coupled through a conductor comprising pad  42  and via  46  to a second internal conductive layer  33 . Via  46  extends from surface  31  of PCB  3  to conductive layer  33 . In the present embodiment of the invention, conductive layer  33  is the return current path for the voltage supply V+. Although the voltage potential of conductive layer  33  need not necessarily be at earth potential, layer  33  is a system ground plane and, for purposes of this Description, will be referred to as GND. 
     With respect to C 2 , recall that C 2  is laterally juxtaposed (in the manner defined above) to C 1  on surface  31  of PCB  3 . As may be seen in FIG. 1D, the first conductive terminal  22  of capacitor C 2  is electrically coupled through a conductor comprising pad  43  and via  47  to the second internal conductive layer  33  (GND). Via  47  extends from surface  31  of PCB  3  to the ground plane GND. Similarly, the second conductive terminal  23  of C 2  is electrically coupled through a conductor comprising pad  44  and via  48  to internal lay  32 , that is the power plane V+. 
     As a result of the physical arrangement of C 1  and C 2  on PCB  3 , and the electrical connection between C 2  and C 3 , respectively, and V+ and GND, respectively, as effected by the conductor array comprising pads  41 ,  42 ,  43 , and  44  and vias  45 ,  46 ,  47 , and  48 , C 1  and C 2  are electrically connected in parallel across V+ and GND. This minimizes the effect of parasitic lead inductance, as explained above. However, the discharge currents that flow in C 1  and C 2  as a result of, for example, logic state transitions, traverse in opposite directions. Accordingly, the flux lines surrounding C 1  and C 2  likewise travel in opposite directions. See FIG.  1 D. Consequently, the mutual inductance between C 1  and C 2  is substantially increased. This in turn, reduces the parasitic inductance, ideally to zero, and thereby decreases the radiated field, likewise ideally to zero. 
     A second embodiment of the invention is depicted in FIG.  2 . In that arrangement, decoupling capacitor C 1  remains positioned on, and affixed to, first surface  31  of PCB  3 . However, rather than the lateral juxtaposition of C 2  illustrated in FIG. 1, C 2  is now positioned on, and affixed to, a second exterior surface  34  of PCB  3 . In this configuration, C 1  and C 2  may be said to be orthogonally juxtaposed about PCB  3 . That is, if each of the decoupling capacitors is assumed to be characterized by a second axis that extends through the centers of the respective capacitors, in a direction orthogonal to their respective first axes (and substantially orthogonal to surfaces  31  and  34  of PCB  3 ), then, in the context of this Description, orthogonal alignment means that the respective second axes of C 1  and C 2  are collinear. 
     As may be seen in FIG. 2, the first conductive terminal  12  of capacitor C 1  is electrically coupled through the conductor comprising pad  41  and via  45  to the first internal conductive layer  32 . Via  45  extends from surface  31  of PCB  3  to conductive layer  32 . Similarly, the second conductive terminal  13  of C 1  is electrically coupled through the conductor comprising pad  42  and via  46  to the second internal conductive layer  33 . Via  46  extends from surface  31  of PCB  3  to conductive layer  33 . 
     With respect to C 2 , recall that C 2  is orthogonally juxtaposed (in the manner defined above) to C 1  on surface  34  of PCB  3 . As may be seen in FIG. 2, the first conductive terminal  22  of capacitor C 2  is electrically coupled through a conductor comprising pad  43  and via  47  to the second internal conductive layer  33  (GND). Via  47  extends from surface  31  of PCB  3  to the ground plane GND. Similarly, the second conductive terminal  23  of C 2  is electrically coupled through a conductor comprising pad  44  and via  48  to internal lay  32 , that is, the power plane V+. 
     As a result of the physical arrangement of C 1  and C 2  on PCB  3 , and the electrical connection between C 2  and C 3 , respectively, and V+ and GND, respectively, as effected by the conductor array comprising pads  41 ,  42 ,  43 , and  44  and vias  45 ,  46 ,  47 , and  48 , C 1  and C 2  are electrically connected in parallel across V+ and GND. As with the arrangement depicted in FIGS. 1A,  1 B, and  1 C, this minimizes the effect of parasitic lead inductance, as explained above. However, the discharge currents that flow in C 1  and C 2  as a result of, for example, logic state transitions, traverse in opposite directions. Accordingly, the flux lines surrounding C 1  and C 2  likewise travel in opposite directions. Consequently, the mutual inductance between C 1  and C 2  is substantially increased. This, in turn, reduces the parasitic inductance, ideally to zero, and thereby decreases the radiated field, likewise ideally to zero. 
     A third embodiment of the invention is depicted in FIG.  3 . In that embodiment, capacitors C 1  and C 2  are axially aligned, in an end-to-end relationship on exterior surface of  31  of PCB  3 . That is, C 1  and C 2  are positioned on surface  31  of PCB  3  so that their respective first axes (as that term is defined above) are collinear. 
     As may be seem in FIG. 3, the first conductive terminal  12  of capacitor C 1  is electrically coupled through the conductor comprising pad  41  and via  45  to the first internal conductive layer  32 . Via  45  extends from surface  31  of PCB  3  to conductive layer  32 . Similarly, the second conductive terminal  13  of C 1  is electrically coupled through the conductor comprising pad  42  and via  46  to the second internal conductive layer  33 . Via  46  extends from surface  31  of PCB  3  to conductive layer  33 . 
     With respect to C 2 , notice that C 2  is axially aligned (in the manner defined above) to C 1  on surface  31  of PCB  3 . As may be seen in FIG. 3, the first conductive terminal  22  of capacitor C 2  is electrically coupled through a conductor comprising pad  43  and via  47  to the second internal conductive layer  33  (GND). Via  47  extends from surface  31  of PCB  3  to the ground plane GND. Similarly, the second conductive terminal  23  of C 2  is electrically coupled through a conductor comprising pad  44  and via  48  to internal lay  32 , that is, the power plane V+. 
     As a result of the physical arrangement of C 1  and C 2  on PCB  3 , and the electrical connection between C 2  and C 3 , respectively, and V+ and GND, respectively, as effected by the conductor array comprising pads  41 ,  42 ,  43 , and  44  and vias  45 ,  46 ,  47 , and  48 , C 1  and C 2  are electrically connected in parallel across V+ and GND. As with the arrangement depicted in FIGS. 1 and 2, this minimizes the effect of parasitic lead inductance, as explained above. However, in contradiction to the arrangements of the embodiments described above, the discharge currents that flow in C 1  and C 2  as a result of, for example, logic-state transitions, traverse those capacitors in the same direction. Accordingly, the flux lines surrounding C 1  and C 2  likewise surround those capacitors in the same direction. However, the currents flowing through vias  45 ,  46 ,  47  and  48  give rise to mutual inductances between those vias. Consider for example, the mutual inductance between via  46  and via  48 . 
     The general expression for mutual inductive coupling between two vias is:          M   =         μ   0       2      π          l        {       ln        [       l   d     +           (     1   d     )     2     +   1         ]       +     d   l     -           (     d   l     )     2     +   1         }         ,                          
     where “1” is the length of the via and “d” is the center-to-center distance between the vias, in meters, and μ hd 0  is the permeability of free space and is equal to 4π×10 7  henry/meter. Therefore, the voltage drop across via  46  is given by the expression: 
     
       
         V 46 =sLI 46 −sMI 48 , 
       
     
     where L is the inductance of the via (see above), I 46  is the current flowing in via  46 , and I 48  is the current flowing in via  48 . When I 46 =I 48 , then V 46=s(L−M)I   46 . In this manner, it has been found that the effects of parasitic inductance associated with the via can be reduced, by approximately 10%. 
     Although the subject invention has been described with respect to the specific exemplary embodiments disclosed above, the invention is not limited to those embodiments. Various modifications, improvements, and additions may be implemented by those skilled in the art; and such modifications, improvements, and additions are comprehended within, or are to be deemed equivalent to, the scope of the appended Claims. For example, the invention is disclosed in the context of multilayer PCBs. However, the structure of the PCB is not a requisite element of the invention, which has applicability to other PCB structures. Furthermore, the invention is described as embracing laterally juxtaposed, orthogonally juxtaposed and axially aligned capacitor pairs. However, the scope of the invention is scaleable to arrangements consisting of more than two capacitors and to orientations constituting combinations of the orientations specifically disclosed. In addition, although the invention is described in the context of PCBs that implement conductive arrays comprising conductive terminals inherent to the decoupling capacitors, as well as pads and vias that may be viewed as elements of the PCB, those having ordinary skill in the art readily appreciate that the inventive concept disclosed hererin extends to conductive arrays of other forms, and PCB pads and vias represent only a specific instantiation of the invention.