Abstract:
One embodiment of the invention comprises an improved method for making a via structure for use in a printed circuit board (PCB). The via allows for the passage of a signal from one signal plane to another in the PCB, and in so doing transgresses the power and ground planes between the signal plane. To minimize EM disturbance between the power and ground planes, signal loss due to signal return current, and via-to-via coupling, the via is shielded within two concentric cylinders, each coupled to one of the power and ground planes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional of U.S. patent application Ser. No. 14/010,051, filed Aug. 26, 2013, which is a divisional of U.S. patent application Ser. No. 13/190,597, filed Jul. 26, 2011, now issued as U.S. Pat. No. 8,516,695, which is a continuation of U.S. patent application Ser. No. 12/699,428, filed Feb. 3, 2010, issued as U.S. Pat. No. 7,992,297, which is a divisional of U.S. patent application Ser. No. 11/533,005, filed Sep. 19, 2006, issued as U.S. Pat. No. 7,676,919, which is a divisional of U.S. patent application Ser. No. 11/114,420, filed Apr. 26, 2005 (abandoned). Priority is claimed to these applications, and all are incorporated herein by reference in their entireties. Furthermore, this application relates to U.S. Pat. No. 7,459,638, entitled “Absorbing Boundary for a Multi-Layer Circuit Board Structure,” which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of this invention relate to printed circuit boards, and in particular to an improved via structure for providing signal integrity improvement. 
       BACKGROUND 
       [0003]    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 the 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. 
         [0004]    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 thro ugh 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. 
         [0005]    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 . 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. 
         [0006]      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  16  (with a dielectric constant of 4.2). 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. 
         [0007]      FIG. 2  shows the transmission coefficient of the aggressor signal, and significant signal 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. 3  shows the coupling coefficient between the aggressor and victim signals. As can be seen, the coupling coefficient stands close to −10 db around all resonance frequencies, indicating significant cross-talk between the aggressor and the victim. 
         [0008]    The prior art has sought to remedy these problems in a number of different ways. First, as disclosed in Houfei Chen et al., “Coupling of Large Numbers of Vias in Electronic Packaging Structures and Differential Signaling,” IEEE MTT-S International Microwave Symposium, Seattle, Wash., Jun. 2-7 (2002), it was taught to surround vias of interest in a PCB with shielding vias. In U.S. Pat. No. 6,789,241, it was taught to place decoupling capacitors between the power and ground planes on a PCB at different locations. In Thomas Neu, “Designing Controlled Impedance Vias,” at 67-72, EDN (Oct. 2, 2003), it was taught to minimize the impedance discontinuity caused by the via structure by adding four companion vias, all connected to ground planes. All of these references cited in this paragraph are hereby incorporated by reference. 
         [0009]    However, these prior approaches suffer from drawbacks, as will be discussed in further detail later. In any event, 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 improved, shielded via structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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: 
           [0011]      FIG. 1A  illustrates a perspective view of two prior art vias both switching signal planes through power and ground planes. 
           [0012]      FIG. 1B  illustrates a cross section of one of the vias of  FIG. 1A . 
           [0013]      FIG. 2  illustrates signal loss (via S-parameters) as a function of frequency for both the prior art via of  FIG. 1B  and the disclosed via of  FIG. 4 . 
           [0014]      FIG. 3  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. 4 . 
           [0015]      FIG. 4  illustrates a cross section of the disclosed improved via structure. 
           [0016]      FIGS. 5A-5N  illustrate sequential steps for the construction of the via of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 4  shows an improved via structure  50  which alleviates the problem of signals switching signal planes through power planes. 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 . Also similarly to  FIG. 1B , power and ground planes  62  and  64  are present. However, in distinction to  FIG. 1B , the power and ground planes  62  and  64  are coupled to concentric cylinders  62   a  and  64   a  (i.e., shields) around the via  60 . Through this configuration, the cylinders  62   a,    64   a  substantially encompasses the via in directions perpendicular to its axis  61 , such that the cylinders are positioned in a dielectric perpendicularly to the plane of the PCB  66 . 
         [0018]    This via structure  50  facilitates signal transitioning from one plane to another by reducing the disturbances cause by return path discontinuities, particularly at high frequencies. Moreover, the via structure  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 approach provides more efficient via shielding than the use of shielding vias, discussed in the background. Moreover, the disclosed approach performs better at high frequency than do approaches using decoupling capacitors, which otherwise suffer from relatively high effective series inductances that exist in decoupling capacitors, again as discussed in the background. As compared to prior art seeking to minimize the impedance discontinuity caused by the via, also discussed in the background, the disclosed approach is more flexible and realistic. In that prior art approach, both of the planes transgressed must be held at the same potential (i.e., ground or power). In short, that prior technique has no pertinence when signals have to change through both power and ground planes, as that technique would require shorting those planes together, which is not possible in a real working PCB. In short, it provides no solution for the problem addressed here of switching through power and ground planes. In short, the disclosed via structure has improved applicability to high-speed/high-frequency PCB designs, where signals have reduced timing and noise margins and increased energies. 
         [0019]    The improved performance is shown in  FIGS. 2 and 3 , which as discussed previously shows computer simulation results indicative of the magnitude of the EM disturbance caused by signal plane switching and cross-talk. Thus, referring again to  FIG. 2 , it is seen that the disclosed via structure  50  has an improved transmission coefficient (i.e., S-parameter), and does not generally suffer large “dips” in the transmission coefficient resulting from unwanted resonance in the cavity between the power planes. Moreover, and referring again to  FIG. 3 , it can be seen that cross-talk is greatly minimized, especially at higher frequencies. As modeled, the core via of  FIG. 4  had the same core dimensions and materials of the via of  FIG. 1  as discussed in the background, and had the following additional parameters: an inner power diameter  56  of 20 mils; an inner ground diameter  58  of 23 mils; cylinder wall thicknesses of 2 mils; a 1 mil dielectric thickness  57  between the cylinders; and a 3 mil vertical distance  55  between the top of the power cylinder  64   a  and the ground plane  62 . (As such, it should be understood that the cross section of  FIG. 4  is not drawn to scale). Of course, these values for the improved via structure  50  are merely exemplary, and can be changed depending on the environment in which the vias will operate. For example, the core via  60  can be made of a smaller diameter, and the cylinders  62   a,    64   a  can be further spaced from core via  60 . 
         [0020]    As shown in  FIG. 4 , it is preferable to place the power and ground cylinders  62   a,    64   a  as close as together to maximize the coupling between them. Preferably, the dielectric thickness  57  between the cylinders would not exceed 3 mils for the materials discussed herein. 
         [0021]    Although the via structure  50  is shown in  FIG. 4  with the power cylinder  62   a  within the ground cylinder  64   a,  it should be understood that the cylinders can be reversed with the same effect, i.e., with the ground cylinder  64   a  within the power cylinder  62   a.    
         [0022]    Manufacture of the disclosed via structure  50  can take place as illustrated in the sequential cross-sectional views of  FIG. 5A-5N . Most of the individual steps 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 at www.ul.ie/˜rinne/ee6471/ee6471%20wk11.pdf, which is incorporated herein by reference in its entirety, and which is submitted with the Information Disclosure Statement filed with this application. 
         [0023]    Starting with  FIG. 5A , the starting substrate comprises a dielectric layer  66  which has been coated on both sides with a conductive material  62 ,  64 , which comprises the power and ground planes. In a preferred embodiment, dielectric  66  is FR4, but could comprise any dielectric useable in a PCB. The conductive materials  62 ,  64  can also comprise standard PCB conductive materials. 
         [0024]    In  FIG. 5B , a hole  70  that will eventually encompass the cylinders is formed. Such a hole  70  can be formed by mechanical or laser drilling. Note that the hole  70  does not proceed through the entirety of the dielectric  66 , but instead leaves a thickness akin to the thickness  55  ( FIG. 4 ) in the finished via. 
         [0025]    In  FIG. 5C , the resulting structure is electrically plated to form line  71  the hole  70 . Processes for electrical plating are well known in the art, and hence are not further discussed. Note that through this process the plating  71  couples to the ground plane  64 . In  FIG. 5D , the horizontal portion of the plating  71  is removed, which can occur using plasmas or wet chemical etchants. In this regard, it may be useful to employ a removable masking layer (not shown) over conductors  62 ,  64  to protect them against the etch step of  FIG. 5D , which would then allow an anisotropic plasma etch to be used to remove only the horizontal portion of the plating  71 . The resulting structure defines the outer cylinder  64   a.    
         [0026]    In  FIG. 5E , the hole  70  is filled with another dielectric material  72 . This dielectric material can be deposited either by chemical vapor deposition, or “spun on” to the substrate in liquid form and then hardened. Either way, the bottom side of the substrate might need to be planarized to remove unwanted portions of the dielectric material  72  from the surface of the ground plane  64 . 
         [0027]      FIGS. 5F-5I  essentially mimic the steps of  FIGS. 5B-5E  (drilling, plating, etching, and dielectric filling), but occur on the top of the substrate and are relevant to the formation of the inner cylinder (i.e.,  62   a ). As these steps are the same, they are not again discussed. 
         [0028]    In  FIG. 5J , sheets of a dielectric prepreg material  78  are adhered to the top and bottom of the substrate. The prepreg sheets  78  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. 
         [0029]    In  FIG. 5K , a conductive material  80  for the signal traces is formed on both the top and bottom of the substrate. Again, plating and/or chemical vapor deposition can be used to form the conductive material  80 . 
         [0030]    In  FIG. 5L , a hole  82  for the via is formed. Such hole may be mechanically drilled or formed by laser drilling. 
         [0031]    In  FIG. 5M , another conductive material  84  is placed on the sides of the hole  84  to form via  80 , e.g., by plating and/or chemical vapor deposition. In so doing, the conductive material  84  contacts the top and bottom conductive material  80  deposited in  FIG. 5K . 
         [0032]    In  FIG. 5N , the conductive material  80  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, top and bottom conductors  80   t,    80   b  are formed, thus forming, in conjunction with the via  80 , a signal which switches signal planes through the power planes, i.e., the problematic configuration discussed above. However, the dual-shield configuration minimizes the effects of EM disturbance. 
         [0033]    The disclosed via structure  50  is susceptible to modifications. It is preferable that the shields  62   a,    64   a  are circular and concentric, as this geometry is easiest to manufacture. However, useful embodiments of the invention need not be either circular or concentric. For example, the shields  62   a,    64   a  can take the form of squares, rectangles, ovals, etc., and additionally need not be perfectly concentric to achieve improved performance. The dielectric material ( 72 ;  FIG. 5E ) between the cylinders  62   a,    64   a  need not be FR4, but could comprise other high dielectric constant materials other than those mentioned. Finally, the number of shields can be increased. Thus, there could be three shields (e.g., with a ground shield nested between two power shields or vice versa), four shield (with alternating power and ground shields), or more. 
         [0034]    Although particularly useful in the context of a printed circuit board, the disclosed technique could also be adapted to the formation of shielded vias for integrated circuits. 
         [0035]    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.