Abstract:
The invention comprises an improved PCB board design having particular utility for high frequency application, and especially useful to alleviate the problem of electromagnetic disturbance of signals switching through power and ground planes. In one embodiment, the PCB contains a magnetically loaded absorbing boundary to absorb the EM disturbances and keep them from resonating inside the cavity between the power and ground planes. The boundary is preferably placed at an edge or edges of the PCB, where it is unlikely to affect any other signals on the PCB. Exemplary materials for the boundary have a magnetic loss tangent of 1.0 to 1.5 with an attenuation constant of −20 dB/cm over frequencies of interest. Depending on whether the boundary material is solid or non-solid, it may be adhered to the edges of the PCB, or may be applied to the edge and cured. It is preferable that the boundary material span through substantially the entire height of the dielectric cavity between the power and ground planes to best absorb the electromagnetic disturbances.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application relates to an application Ser. No. 11/114,420 entitled “Circuit Board Via Structure for High Speed Signaling,” which is filed concurrently with the U.S. Patent &amp; Trademark Office, and which is incorporated herein by reference in its entirety. 
   FIELD OF THE INVENTION 
   Embodiments of this invention relate to printed circuit boards, and in particular to an improved design for minimizing electromagnetic disturbance using a magnetically-loaded absorbing boundary. 
   BACKGROUND 
   In a multilayer printed circuit board (PCB), there are occasions that signals have to switch signaling planes in the PCB.  FIGS. 1A and 1B  illustrate such signal plane switching. As best shown in the cross sectional view of  FIG. 1B , a signal trace  18   t  originally proceeding on top of a PCB  15  meets with a via  18  appearing through the PCB  15  and down to another signal trace  18   b  on the bottom of the PCB  15 . Thus, by use of the via  18 , the signal trace is allowed to change planes in the printed circuit board, which can facilitate signal routing. 
   Also present in the PCB  15  are power (i.e., Vdd) and ground planes, respectively numbered as  12 ,  14 , and referred to collectively as “power planes.” These power planes  12 ,  14  allow power and ground to be routed to the various devices mounted on the board (not shown). (Although shown with the power plane  14  on top of the ground plane  12 , these planes can be reversed). When routing a signal through these power planes, it is necessary to space the via  18  from both planes  12 ,  14 , what is referred to as an antipad diameter  12   h,    14   h.  The vias themselves at the level of the signal planes have pads to facilitate routing of the signals  18   t,    18   b  to the via, which have a pad diameter ( 18   p ) larger than the diameter of the via  18  itself (d). Typical values for the diameter of the via (d), the pad diameter ( 18   p ) and the antipad diameter ( 12   h,    14   h ) are 16, 20, and 24 mils respectively. It should be understood that an actual PCB  15  might have several different signal and power planes, as well as more than two signal planes, although not shown for clarity. 
   When a signal trace such as  18   t,    18   b  switches signal planes, the signal return current—a transient—will generate electromagnetic (EM) waves that propagate in the cavity  17  formed between the power and ground planes  12 ,  14 . More specifically, when a signal trace switches planes, if there is no decoupling capacitance around the switching point, a high frequency return current will proceed through the capacitance formed by the power and ground planes  12 ,  14 . Due to the limited size and open boundary of the PCB  15 , the cavity  17  between the planes  12 ,  14  is induced with EM waves. Such EM waves will cause electrical disturbance on the signal being switched, as well as other signals traces. Such disturbances are especially felt in other near-by signals traces that are also switching signal planes, such as signal traces  16   t,    16   b  ( FIG. 1A ) due to coupling between the vias (i.e.,  18  and  16 ). Moreover, such EM disturbances are significantly enhanced around the resonant frequencies of the power/ground cavity  17 , which in turn are determined by the physical dimensions of the power planes  12 ,  14 . Via-to-via coupling induced by signal plane switching can cause significant cross-talk, and can be particularly problematic for high frequency switching applications. 
     FIGS. 2 and 3 , representing computer simulations on the structure of  FIG. 1A , illustrate these problems. In these simulations, one of the signal lines (say, signal  16 ) is an “aggressor” through which a simulated signal is passed, and the other signal line (signal  18 ) is the “victim” whose perturbation is monitored. The simulations were run in HFSS™, which is a full-wave three-dimensional EM solver available from Ansoft Corporation of Pittsburgh, Pa. The simulations were run assuming a 2.0-by-0.4 inch PCB  15 , a spacing of 100 mils between the two vias  16 ,  18 , a height of 54 mils between the power planes  12 ,  14  defining the cavity  17 , and use of an FR4 dielectric for the PCB  15  (with a dielectric constant of 4.2 and a loss tangent of 0.02). Traces  16   t,    16   b,    18   t,  and  18   b  were assumed to be microstrip lines with a characteristic impedance of 40 ohms. Via diameters, via pad diameters, and antipad diameters were assumed to have the values mentioned previously. 
     FIGS. 2 and 3  respectively show the transmission and reflection coefficients of the aggressor signal, and significant signal insertion and return loss is observed around certain resonant frequencies. The measured parameter is a scattering parameter (S-parameter), which is a standard metric for signal integrity and which is indicative of the magnitude of the EM disturbance caused by signal plane switching.  FIG. 4  shows the coupling coefficient between the aggressor and victim signals. As can be seen, the coupling coefficient stands close to −8 dB around certain resonant frequencies, indicating significant cross-talk between the aggressor and the victim. If the signals used on the PCB  15  have component frequencies at or near these resonance peaks, circuit performance and signal integrity could be compromised. 
   The prior art has sought to remedy these problems in a number of different ways. In U.S. Pat. No. 6,789,241, incorporated herein by reference, it was taught to place decoupling capacitors between the power and ground planes on a PCB at different locations. But such a solution is not optimal, due to significantly high effective series inductances that exist in decoupling capacitors at high frequencies. In U.S. Pat. No. 6,441,313, also incorporated herein by reference, it was taught to use a dielectric material between the power and ground planes with a high dielectric loss tangent. However, such materials exhibit loss tangents on the order of 0.2 to 0.3, which is too low to significantly mitigate the EM disturbance problem discussed above. Moreover, because the &#39;313 patent applies the high loss tangent material through the entirety of the cavity between the power and ground planes on the PCB, it will inevitably adversely affect any stripline structures between those two planes. 
   Thus, the art would be benefited from strategies designed to minimize problems associated with signals switching signal planes in a printed circuit board. This disclosure provides such a solution in the form of an EM-absorbing boundary material applied at the edges of the PCB. 
   SUMMARY 
   The invention comprises an improved printed circuit board design having particular utility for high frequency application, and especially useful to alleviate the problematic electromagnetic disturbance of signals switching through power and ground planes. In one embodiment, the PCB contains a magnetically loaded absorbing boundary to absorb the EM disturbances and keep them from resonating inside the cavity between the power and ground planes. The boundary is preferably placed at an edge or edges of the PCB, where it is unlikely to affect any other signals on the PCB. One exemplary material for the boundary is the Series 330-X family of rigid and castable Absorbers, manufactured by Cuming Microwave Corporation, with such material preferably having a magnetic loss tangent of 1.0 to 1.5 with an attenuation constant of −20 dB/cm over frequencies of interest. Depending on whether the boundary material is solid or non-solid, it may be adhered to the edges of the PCB, or may be applied to the edge and cured. It is preferable that the boundary material span through substantially the entire height of the dielectric cavity between the power and ground planes to best absorb the electromagnetic disturbances. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which: 
       FIG. 1A  illustrates a perspective view of two prior art vias both switching signal planes through power and ground planes. 
       FIG. 1B  illustrates a cross section of one of the vias of  FIG. 1A . 
       FIGS. 2 and 3  respectively illustrates signal loss and signal reflection (via S-parameters) as a function of frequency for both the prior art via of  FIG. 1B  and the disclosed via of  FIG. 5 . 
       FIG. 4  illustrates via coupling (in dB) as a function of frequency for both the prior art via of  FIG. 1B  and the disclosed via of  FIG. 5 . 
       FIG. 5  illustrates a cross section of the disclosed improved PCB design having an absorbing boundary material at the edges of the PCB bounding the cavity formed by the power and ground planes. 
       FIGS. 6A-6F  illustrate sequential steps for the construction of a PCB having the absorbing material in which the absorbing material is initially a solid. 
       FIGS. 7A-7E  illustrate sequential steps for the construction of a PCB having the absorbing material in which the absorbing material is initially non-solid. 
   

   DETAILED DESCRIPTION 
     FIG. 5  shows an improved PCB structure  50  for ameliorating the problem of EM disturbances in the cavity  67  between the power and ground planes as discussed earlier. As shown, and similar to  FIG. 1B , a signal  60  switches from the top ( 60   t ) to the bottom ( 60   b ) of the PCB  66  through via  60 , the problematic condition discussed earlier. Also similar to  FIG. 1B , power and ground planes  62  and  64  are present. One skilled in the art will realize that while only one exemplary via  60  is shown, in reality a given cross section of the PCB  50  would likely reveal many vias. Additionally, one skilled will understand that other signal traces will likely be apparent in cross section at the signal planes, and a few exemplary traces  61   t,    61   b  are shown. 
   In distinction to  FIG. 1B , the PCB  50  is bounded by an absorbing boundary  70 , the function of which is to absorb any EM disturbances which might be resonating in the cavity  67  due to signal return current through the via  60 , or due to other parasitic EM disturbances. The absorbing boundary is preferably magnetically loaded, and has a high magnetic loss tangent of 1.0 to 1.5 with an attenuation constant of −20 dB/cm. A magnetically loaded material comprises ferromagnetic particles (e.g., iron, nickel, cobalt) to increase the magnetic loss tangent. This loss tangent is significantly higher than prior art techniques (discussed earlier), and unique in that the material is placed only at the edges of the PCB  50 . Because the absorbing material  70  is positioned at the edges of the PCB  50 , as opposed to in the cavity  67  itself, signal traces ( 60   t,    61   t,    60   b,    61   b ) are not adversely affected. Hence, resonance is absorbed without affecting circuit performance. 
   The use of high loss tangent materials such as absorbing material  70  in a high frequency PCB  50  seems counterintuitive. The loss tangent generally determines the loss of the medium, such that low loss tangents result in a “fast” PCB while large loss tangents result in a “slow” PCB). However, as just noted, by using the absorbing material  70  only at the edges of the PCB  50  (or more specifically, at the edges of the cavity  67 ), general performance of the circuitry can be preserved while attenuating the EM disturbances at the edge of the PCB, where they would otherwise reflect, causing unwanted resonance. 
   To effectively absorb the energy, the absorbing material  70  preferably exhibits high EM attenuation over a broad range of frequencies. In this regard, note that the loss of a dielectric material is determined by its dielectric loss tangent (i.e., the tangent of the complex permittivity: tan(δ d )=∈″/∈′, where ∈=∈′+i∈″) and its magnetic loss tangent (i.e., the tangent of the complex permeability: tan(δ m )=μ″/μ′, where μ=μ′+iμ″). This dualistic nature of loss is reflected in a given material&#39;s “attenuation constant,” which is a function of both the dielectric loss tangent and the magnetic loss tangent. This is explained via equations published on the internet at the URL address originally cited upon the filling of this application, which provides an equation for the attenuation constant as a function of the dielectric loss tangent and the magnetic loss tangent as a function of frequency. These equations are incorporated by reference, and are submitted in the Information Disclosure statement filed with this application. When a magnetically loaded material is used as the absorbing material  70 , iμ″ is very high, and hence the magnetic loss tangent, loss generally, and the attenuation constant specifically, are high as well. 
   The absorbing material  70  preferably exhibits an attenuation constant of at least approximately −20 dB/cm over frequencies or frequency ranges of interest. Suitable materials for the absorbing material  70  can comprise the 330 Series of rigid and castable absorbers, manufactured by Cuming Microwave Corporation of Avon, Mass. Data sheets for several suitable materials (e.g., the 330-X C-RAM family of materials) are posted at on the internet at the URL address originally cited upon the filing of this application, which data sheets are incorporated herein by reference, and are submitted in the Information Disclosure statement filed with this application. 
   However, EM attenuation will be improved, and the problem of signal plane switching improved, even at lower levels for the attenuation constant of the absorbing material  70 . In a broadest form of the invention, all that is required is that the attenuation constant for the absorbing material  70  for a given frequency be higher (in absolute value) than the attenuation constant for the dielectric which makes up the cavity  67  (e.g., FR4) and or the PCB  50 . Or, all that is required is that the loss tangent for the absorbing material  70  for a given frequency be higher than the loss tangent for the dielectric which makes up the cavity  67 . In short, the absorbing material  70  need only be more EM absorbent compared to the dielectric inside the cavity. 
   While it is preferred that the absorbing material  70  have a high magnetic loss tangent, what is ultimately important is that the absorbing material  70  have a higher (e.g., compared to the cavity  67 ) loss, be it by virtue of the dielectric loss tangent or the magnetic loss tangent. Therefore, the disclosed exemplary absorbing materials, which are primarily magnetically loaded, should be understood as merely exemplary. 
   The positioning of the absorbing material  70  can occur in several different ways. As shown in  FIG. 5 , the absorbing material  70  occurs at the edge of the PCB  50 . As shown, the absorbing material  70  can have a thickness which supersedes the power and ground planes  62 ,  64 , but can be made thinner, i.e., approximately or equal the thickness of the cavity  67 , as shown at  70   x  (‘t’). Moreover, the absorbing material  70  can occur on only one side of the PCB  50 , on two opposing sides, or on all four sides, with increasing performance the more sides that are covered. Moreover, the absorbing material  70  need not be continuous on a given side, or at the corners of the PCB  50 , although again this would be preferable to maximize absorption of EM disturbances in the cavity  67 . In any event, for simplicity, in  FIG. 5 , the absorbing material  70  is shown only on two opposing sides of the PCB  50 . 
   This PCB design  50  facilitates signal transitioning from one plane to another by reducing the disturbances caused by current return path discontinuities, particularly at high frequencies. Moreover, the PCB design  50  suppresses via-to-via coupling otherwise caused by resonance between the ground and power planes  62 ,  64  at high frequencies, thereby improving signal integrity and reducing cross-talk from aggressor signals. The improved performance is shown in  FIGS. 2-4 , which as discussed previously shows computer simulation results indicative of the magnitude of EM disturbances caused by signal plane switching and cross-talk between adjacent vias. Thus, referring again to  FIGS. 2 and 3 , it is seen that the disclosed PCB structure  50  has an improved transmission and reflection coefficient (i.e., S-parameter), and does not generally suffer large “dips” resulting from unwanted resonance in the cavity between the power planes. Moreover, and referring again to  FIG. 4 , it can be seen that cross-talk is greatly minimized. As modeled, the PCB  50  of  FIG. 5  was similar to that of  FIG. 1 , but additionally was modeled assuming use of an absorbing material  70  of width (w) 300 mils, with a magnetic loss tangent of approximately 1.0 to 1.5 and an attenuation constant of −20 dB/cm over the broadband of frequencies simulated. 
   Manufacture of the PCB  50  to add the absorbing material  70  can proceed as shown in  FIG. 6 and 7 , which respectively illustrate process flows for uses of absorbing materials  70  which are initially in solid ( 70   a ) or liquid, paste, foam, or epoxy ( 70   b ) forms. Most of the individual steps in the formation of the PCB involve common techniques well known in the PCB arts, and so are only briefly discussed. Further information on such steps are disclosed in “PCB/Overview” (Apr. 11, 2004), which is published on the internet at the URL address originally cited upon the filing of this application, which is incorporated herein by reference in its entirety, and which is submitted with the Information Disclosure Statement filed with this application. 
   Starting with  FIG. 6A , the process of forming a PCB  50  using a rigid absorbing material  70   a,  such as 330-1 C-RAM RGD, is illustrated. As shown, the process begins with the use of a core dielectric cavity material  67 , e.g., FR4 having a loss tangent of 0.02. Absorbing material  70   a,  is cut to size, such that its thickness ‘t’ matches that of the cavity material  67 . The absorbing material  70   a  might have a thickness of 54 mils, and a width of 300 mils, and thus is not drawn to scale. Moreover, absorbing material  70   a  may be formed on all edges of the cavity material  67  (and resulting PCB) although not shown. 
   In  FIG. 6B , the cladding copper for the power and ground planes  62 ,  64  is shown. Adhesive is applied to the various internal surfaces and the materials pushed together as shown in  FIG. 6C  to form an intermediate substrate structure. The thin sheets of absorbing material  70   a  adhere well to the copper cladding  62 ,  64 , resulting in a stable substrate structure. Thermosetting may also be useful at this stage. 
   Thereafter, and referring to  FIG. 6D , sheets of a dielectric prepreg material  68  are adhered to the top and bottom of the substrate. The prepreg sheets  68  are heated and hardened to adhere them to the remaining substrate, which can occur in a hydraulic press. Once adhered, the prepreg forms the dielectric between the power planes/associated cylinders and the signal traces, as will become evident in the following Figures. 
   In  FIG. 6E , a conductive material  72  for the signal traces is formed on both the top and bottom of the substrate. Plating and/or chemical vapor deposition can be used to form the conductive material  72 . Thereafter, and referring to  FIG. 6F , a hole for via  60  is formed, e.g., by mechanical or laser drilling. Another conductive material  73  is placed on the sides of the via hole to form via  60 , e.g., by plating and/or chemical vapor deposition. In so doing, the conductive material  73  contacts the top and bottom conductive material  72  deposited in  FIG. 6E . The conductive material  72  is masked and etched using standard PCB techniques to form the necessary conductors on the top and bottom of the substrate. In particular, and as shown, a signal  72  which switches signal planes through the power planes  62 ,  64 , i.e., the problematic configuration discussed above, although now mitigated by the absorbing material  70   a  at the edges of the cavity  67 . 
     FIGS. 7A-7E  illustrate a PCB process flow in which the absorbing materials  70   b  are initially in liquid, paste, foam, epoxy or other non-solid form, such as 330 Series resin or silicone. Referring first to  FIGS. 7A and 7B , notice that the core dielectric cavity material  67 , e.g., FR4, has been milled (e.g., mechanically or by laser) to form trenches or holes  81  therein. While the holes  81  are shown completely through the cavity material  67 , such holes need not pass completely through. As a first step in the process, shown in  FIGS. 7C and 7D , the holes  81  are filled with the absorbing material  70   b.  Depending on the type of material used, it may be necessary to planarize the PCB after application of the absorbing material  70   b  to remove it from the top and bottom of the PCB. Once in position, the absorbing material  70   b  is cured and hardened. Thereafter, completion of the PCB can occur using the steps shown in  FIG. 6B-6F , arriving at the finished PCB shown in  FIG. 7E . 
   Of course, other modifications to the PCB process to add the absorbing material  70  are possible. For example, the absorbing material  70  can essentially be added to the PCB after it is basically complete. Consider  FIG. 5 . In this cross section, the absorbing material  70  has basically been added to the edge(s) of an otherwise finished PCB, e.g., by adhesive or melting (if a solid absorbing material  70  is used), or by coating the edges (if a non-solid absorbing material  70  is used). In short, one benefit to the use of the absorbing material only at the edges of the PCB is that the same can easily be applied to an already completed PCB to improve EM performance without the need to redesign and/or re-assemble the PCB. It should be noted with reference to  FIGS. 5 ,  6 F, and  7 E that the absorbing material  70  can be external or internal to the PCB (compare  FIGS. 6F and 7E ), can extend along the entirety of the thickness of the edge or can be thinner (see  FIG. 5 ), may or may not come into contact with the power and ground planes (see  FIG. 5 ), and may or may not be laterally spaced from edges of the power and ground planes (as in  FIG. 5 ). All that is required is that the absorbing material  70  be so positioned relative to the power and ground planes to absorb EM radiation resonating in the cavity, and one skilled in the art will recognize that this can occur by placing the absorbing material  70  in several different places at the edge of the PCB, and relative to the power and ground planes. 
   Although particularly useful in the context of a printed circuit board, the disclosed technique could also be adapted to the reduction of EM disturbances in integrated circuits. 
   In short, it should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.