Abstract:
A multilayer board for suppressing unwanted electromagnetic waves and noise includes: a power plane and a ground plane constituting a power distribution network; an electromagnetic wave suppression structure placed on the power plane or the ground plane; and a decoupling capacitor placed on the power plane or the ground plane, wherein the electromagnetic wave suppression structure and the decoupling capacitor are placed together.

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATION(S) 
       [0001]    The present invention claims priority of Korean Patent Application No. 10-2009-0128317, filed on Dec. 21, 2009, which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a technique for suppressing unwanted electromagnetic waves and noise generated in a multilayer board; and more particularly, to a multilayer board for suppressing unwanted electromagnetic waves and noise, by applying both decoupling capacitors (DeCaps) and an electromagnetic wave suppression structure including an electromagnetic bandgap (EBG) that are partially placed only at specific areas, such as in the vicinity of noise generating devices and/or noise-sensitive parts, on a power plane or ground plane in a multilayer board. 
       BACKGROUND OF THE INVENTION 
       [0003]    Recently, as wired/wireless broadcasting and telecommunication-related technologies and services have been rapidly developed, and thus the level of users&#39; demand for products has been increasing, advanced information communication equipment and systems are being equipped with various functions and becoming smaller in size so as to be easily carried. To implement this, high-speed digital systems are becoming faster and wider in bandwidth. As the clock frequency falls within the range of several GHz with such increase in the operating speed of the advanced equipment and systems, the problem of signal/power integrity and electromagnetic interference, which is caused by simultaneous switching noise (SSN) generated in a multilayer package or in a multilayer PCB structure, is considered as one of the most important issues in designing the chip/package and PCB of a high-speed system. 
         [0004]    First, the multilayer PCB and package structure will be described. In the multilayer PCB and package structure, generally, a power plane and a ground plane constituting a power distribution network (PDN) are paired and placed inside the multilayer structure, which form a parallel plate waveguide configuration. Shown in  FIG. 1  is a mechanism in which noise is generated in a PDN including power and ground planes due to layer arrangement, signal flow, and a high-speed switching device, such as an IC chip, in a multilayer PCB and package structure using a high-speed signal. 
         [0005]      FIG. 1  is a view showing a signal flow and noise generation mechanism in a multilayer PCB and package structure using a high-speed signal. 
         [0006]    Referring to  FIG. 1 , simultaneous switching noise (SSN)  102  is known to be the most serious noise in a multilayer PCB and chip/package structure. The SSN  102 , also referred to as Delta-I noise or ground bounce noise (GBN), is generated by time-varying currents that change fast in a high-speed digital circuit. The SSN  102  generated between the power plane and a ground plane affects the signal/power integrity of the circuits and also causes unwanted electromagnetic interference (EMI)  104  to be radiated from the edges of a PCB board. Thus, the SSN  102  is becoming an important issue in high-speed digital systems operating at a low voltage level at a high-speed clock frequency. 
         [0007]    A recent high-speed digital system has several hundreds of input/output gates for simultaneous switching. If a fast current flows through vias in the multilayer PCB/package due to simultaneous switching of the large number of gates, unwanted noise (SSN  102 ) is generated between the power plane and ground plane as shown in  FIG. 1 , and the generated SSN  102  is propagated across the PCB/package by a resonance mode of a parallel conducting plate and then unwanted EMI  104  is radiated from the edges of the PCB/package. That is, the SSN  102  is inductive noise generated when many output terminals of the digital circuit simultaneously switch. Since the amount of the SSN  102  depends on the configuration and current path of the PCB/package, it is difficult to measure a precise amount of the noise. However, the noise can be represented most simply by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     noise 
                   
                   = 
                   
                     
                       N 
                       · 
                       
                         L 
                         eq 
                       
                     
                      
                     
                       
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                         i 
                       
                       
                          
                         t 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
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         [0008]    wherein V noise  is a noise voltage, N is the number of simultaneously switching gates, and L eq  is an inductance value caused by current flowing through each driver during simultaneous switching. 
         [0009]    So far, one of the most typical methods to solve the problem of signal/power integrity or EMI generated by SSN in analog and digital systems is to mount a device having a large capacitance, which is called a decoupling capacitor (DeCap), between the power layer and the ground layer. Research for eliminating a parasitic inductance component of the power distribution network (PDN) and properly supplying power to an integrated circuit device by the decoupling capacitor has been continuously conducted. However, the mounting of the DeCap on the PCB increases production costs, and also occupies the space of the PCB board, thus making the placement of various devices restrictive. Also, the parasitic inductance component of the DeCap may cause another parallel resonance frequency. Due to the parasitic inductance, the DeCap can operate only up to several hundreds of MHz, and thus the SSN having a GHz frequency component, which has become a problem in recent high-speed digital systems, cannot be eliminated. 
         [0010]    The most frequently used method in efforts to reduce the parasitic inductance component of the DeCap is an embedded thin film capacitor that has a thin film material having a high dielectric constant disposed between power and ground planes. The use of the embedded thin film capacitor makes SSN reduction characteristic improve even in a higher frequency band than that of the DeCap. However, the embedded thin film capacitor also has a limited frequency band of several hundreds of MHz for use, and in order to put the embedded thin film capacitor to practical use, additional research on the composition of a material having a high dielectric constant and processing techniques using the same is required. 
         [0011]    Besides, various methods, such as stitching vias, ground filling, and the like, have been proposed, but most of the methods are disadvantageous in that they operate locally in limited areas rather than across the substrate and show SSN suppression characteristics only in a narrow frequency band less than GHz, and thus it is known their effects are known to be insignificant in the current high-speed systems. 
         [0012]    Meanwhile, new methods for solving the problems caused by SSN in a GHz band are being studied, and research is ongoing to reduce EMI by eliminating SSN in a chip/package and multilayer PCB structure and thus improving power integrity/signal integrity (PI/SI), by using an electromagnetic bandgap (EBG) structure highly applicable as an EMI reduction technique in a GHz band, that is, an EBG structure having a high impedance characteristic in a specific frequency band to provide a wide bandgap characteristic for currents flowing on surfaces. Reducing SSN by using an EBG in the multilayer PCB/package structure allows more effective PI/SI reduction and EMI suppression than using a DeCap or embedded thin film capacitor, and shows more excellent characteristics in selecting a frequency band to be suppressed. 
         [0013]    However, a mushroom-type EBG structure formed in a double-layer structure has disadvantages that it is difficult to manufacture blind vias and the like in terms of process steps and additional costs are required. To overcome this problem, there has been suggested a single-plane EBG structure using periodic structure of an appropriate pattern on a ground plane or power plane. Although this structure can attain considerable noise reduction in a power distribution network (PDN) having a parallel plate waveguide configuration, it is disadvantageous in that it affects high-speed signals flowing over the ground/power planes to which the EBG structure is applied, and there is a limitation on the lowest frequency of a frequency band to be suppressed. Moreover, in case where an EBG structure is provided only on the ground plane or power plane, self-impedance at the region where the EBG structure is placed, especially in a low frequency band, increases, and thus the generation of unwanted electromagnetic waves becomes more dominant. 
       SUMMARY OF THE INVENTION 
       [0014]    In view of the above, the present invention provides a multilayer board for suppressing unwanted electromagnetic waves and noise, which can apply both decoupling capacitors (DeCaps) and an electromagnetic wave suppression structure including an electromagnetic bandgap (EBG) that are partially placed only in specific areas, such as in the vicinity of noise generating devices and/or noise-sensitive parts, on a power plane or ground plane in a multilayer board. 
         [0015]    Further, the present invention provides a multilayer board for suppressing unwanted electromagnetic waves and noise, which can widen a suppression frequency band from DC up to several tens of GHz (e.g., DC to 99 GHz) and minimize the effects on signals, while maintaining the characteristic of suppressing unwanted electromagnetic waves and noise, by disposing both an electromagnetic wave suppression structure and DeCaps, the electromagnetic wave suppression structure including an EBG used to suppress unwanted wideband electromagnetic waves or noise, such as SSN, generated in a multilayer board structure of electromagnetic devices and systems using a high-speed signal. 
         [0016]    In accordance with the embodiment of the present invention, there is provided a multilayer board for suppressing unwanted electromagnetic waves and noise, including: 
         [0017]    a power plane and a ground plane constituting a power distribution network; 
         [0018]    an electromagnetic wave suppression structure placed on the power plane or the ground plane; and 
         [0019]    a decoupling capacitor placed on the power plane or the ground plane, wherein the electromagnetic wave suppression structure and the decoupling capacitor are placed together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The objects and features of the present invention will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
           [0021]      FIG. 1  is a view showing a signal flow and noise generation mechanism in a multilayer board structure using a high-speed signal; 
           [0022]      FIGS. 2A and 2B  are views showing the placement of electromagnetic bandgap unit cells and a decoupling capacitor applied onto a single plane in accordance with an embodiment of the present invention; 
           [0023]      FIG. 3  shows a cross-sectional structure in a circuit model of a power plane and ground plane in which an electromagnetic bandgap structure and a decoupling capacitor structure are partially placed on a single plane in accordance with the embodiment of the present invention; 
           [0024]      FIG. 4  is a graph showing comparison results of the noise suppression characteristics of the partially placed electromagnetic bandgap and decoupling capacitor structure in accordance with the embodiment of the present invention; 
           [0025]      FIG. 5  is a graph showing the noise suppression characteristics of the partially placed electromagnetic bandgap and decoupling capacitor structure in accordance with the embodiment of the present invention; 
           [0026]      FIGS. 6A to 6C  show a structure in which an electromagnetic wave suppression structure is partially applied only to the power plane and a decoupling capacitor is placed only in the vicinity of a noise generating source in accordance with the embodiment of the present invention; 
           [0027]      FIGS. 7A to 7C  show an example of a structure in which an electromagnetic bandgap structure of a different size is partially applied to the ground plane and the power plane, and DeCaps are placed at specific positions around a noise generating source in accordance with the embodiment of the present invention. 
           [0028]      FIGS. 8A and 8B  are views showing a triangular electromagnetic wave suppression structure and a decoupling capacitor placed only in the vicinity of a noise generating source in accordance with the embodiment of the present invention; and 
           [0029]      FIGS. 9A to 9F  are views showing various methods of placing a decoupling capacitor in a multilayer board structure where a partially placed electromagnetic bandgap structure is applied in accordance with the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Hereinafter, the embodiment of the present invention will be described in detail with reference to the accompanying drawings which form a part hereof. 
         [0031]      FIG. 1  has illustrated a mechanism in which noise is generated, due to signal flow and a high speed switching device such as an IC chip, in a power distribution network (PDN) having a parallel plate waveguide shape and being composed of power and ground planes, in a multilayer printed circuit board (PCB) and package structure using a high-speed signal. As for a signal transmission path in the multilayer PCB and package structure of  FIG. 1 , a return current path is established not through the ground plane alone but along a path where the input impedance of each position becomes lower as the frequency of a signal increases. That is, it can be seen that, when a high-speed signal is used, both of the ground plane and the power plane are used as the return current path. 
         [0032]      FIGS. 2A and 2B  show structures of the PDN in which DeCaps and an electromagnetic wave suppression structure including EBG unit cells are arranged, in a multilayer board structure, in accordance with the embodiment of the present invention. Throughout the present invention, the multilayer board includes a multilayer PCB and a multilayer package board. 
         [0033]    In  FIG. 2A , a DeCap  202  and an electromagnetic wave suppression structure  200  are partially placed only in the vicinity of a noise source or noise-sensitive device on the power plane or ground plane. In  FIG. 2B , for comparison of unwanted electromagnetic waves and noise suppression performance, an electromagnetic wave suppression structure  250  is fully placed across the power plane or ground plane of a PDN structure and a DeCap  252  is placed at the same position as the DeCap  202  of  FIG. 2A . 
         [0034]    In general, as the number of EBG structures having the same configuration increases, the electromagnetic wave suppression characteristics gets better but the frequency bandwidth to be suppressed is almost constant. Therefore, from an engineering point of view, once the suppression characteristics required for SSN reduction are determined, the number of EBG unit cells between an electromagnetic noise source and parts to be protected can be set. 
         [0035]    Although different depending on EBG structure, at least two EBG unit cell structures are required in order to obtain suppression characteristics of more than about −30 dB. However, even if one EBG unit cell is placed in the vicinity of a noise source or a noise-sensitive device depending on a suppression frequency band or suppression level required for the system, unwanted electromagnetic waves and noise suppression characteristics can be obtained. 
         [0036]    Moreover, the generation and transmission of unwanted electromagnetic waves can be suppressed even in a frequency band less than several hundreds of MHz by using the DeCap as well as by partially placing the electromagnetic wave suppression structure. P 1  to P 4  shown in  FIGS. 2A and 2B  are ports used in simulation or measurement in order to show the noise suppression characteristics of the electromagnetic wave suppression structure. 
         [0037]      FIG. 3  shows a cross-sectional structure in a circuit model of power plane and ground plane in which an EBG structure  200  and a DeCap structure  202  are partially placed on a single plane as shown in  FIG. 2A , in accordance with the embodiment of the present invention. 
         [0038]    The present invention is intended to apply both an electromagnetic wave suppression structure and DeCap to power plane or ground plane structures used inside a multilayer board of three or more layers. Although  FIG. 3  describes only the power and ground plane structures for convenience of explanation, the proposed unwanted electromagnetic waves and noise reduction structure is applicable to the power distribution network (PDN) including these power and ground planes. That is, the power plane and the ground plane are embedded in pairs even in a multilayer structure of three or more layers, and thus, the proposed structure is also applicable to the multilayer structure of three or more layers. 
         [0039]      FIG. 4  is a graph showing comparison results of the noise suppression characteristics of the partially placed EBG and DeCap structure in accordance with the embodiment of the present invention, which shows the noise transmission and suppression characteristics of the EBG placement structure proposed in  FIGS. 2A and 2B  in the vicinity of the substrate (at positions P 2  to P 4 ) when P 1  is assumed to be a noise source. 
         [0040]    To exhibit the excellence of the noise suppression characteristics of the proposed structure, a simulation was conducted on the noise transmission characteristics in PCB boards having different configurations depending on the placement of an electromagnetic wave suppression structure and the presence or absence of a DeCap. Also, a simulation was conducted on a double-sided PCB only composed of a conductor of the same size, and the result (reference board) was indicated in  FIG. 4 . 
         [0041]    As shown in  FIG. 4 , in case where both of the partially placed electromagnetic wave suppression structure and the DeCap are used, noise is sufficiently suppressed from DC to 5 GHz. That is, sufficient noise suppression characteristics can be obtained only by placing the EBG structure (PEBG w DeCap ( 402 )) in a specific area without fully placing the EBG structure (FEBG w DeCap ( 404 )) across the power/ground planes, and the DeCap can be used to suppress the unwanted noise at the lowest frequency range below the bandgap of EBG, which is the disadvantage of the exiting single-plane EBG structure, to be lowered to DC. Thus, it can be said that the EBG structure with DeCap (PEBG w DeCap ( 402 )) can be sufficiently used as a suppression structure in the entire frequency band where noise may be generated. Moreover, the partially placed EBG structure can minimize the effects on the signals by properly placing a high-speed signal. 
         [0042]      FIG. 5  is a graph showing the noise suppression characteristics of the partially placed EBG and DeCap structure in accordance with the embodiment of the present invention, which shows the noise suppression characteristics of the unwanted electromagnetic waves and noise suppression structure proposed in  FIGS. 2A and 2B  in the vicinity of the substrate (at positions P 2  to P 4 ) when P 1  is assumed to be a noise source. 
         [0043]      FIGS. 6A to 6C  show a structure in which an electromagnetic wave suppression structure is partially applied to the power plane and DeCaps are placed only in the vicinity of a noise generating source in accordance with the embodiment of the present invention. 
         [0044]    Referring to  FIG. 6A , an electromagnetic wave suppression structure  600  and DeCaps  602  are placed together around a noise generating device in a power plane and ground plane structure. Referring to  FIG. 6B , in case where a noise generating device and a noise-sensitive part co-exist in the power and ground plane structure, an electromagnetic wave suppression structure  610  and DeCaps  612  are partially placed together around the noise generating device and another electromagnetic wave suppression structure  610  is additionally placed in the vicinity of the noise-sensitive part. 
         [0045]    Here, the electromagnetic wave suppression structures  610  separately placed in the two areas may have different electromagnetic wave suppression frequency bandwidths in order to widen the noise suppression frequency bandwidth. 
         [0046]    Referring to  FIG. 6C , in case where a noise generating device or noise-sensitive part is formed over a wide area, or the noise-sensitive part is spaced apart from the vicinity of the noise generating device, an electromagnetic wave suppression structure  620  that is partially placed together in the vicinity of the noise generating device is formed over a wider area having more EBG unit cells than the electromagnetic wave suppression structure  610  in  FIG. 6B  is. 
         [0047]    As shown in  FIGS. 6A to 6C , the EBG structure is placed in the vicinity of an electromagnetic wave-sensitive device as well as a noise generating source, thereby improving a noise suppression effect on the corresponding areas. Moreover, although the EBG structure having the same configuration is applied in  FIGS. 6A to 6C , an EBG structure having a different size or configuration may be used in order to widen the suppression frequency bandwidth. 
         [0048]    As can be seen in  FIG. 1 , generally, not only the ground plane but also the power plane is often used as a return current path of high-speed signals. That is to say, since the EBG structure is partially applied, the power or ground plane to which the EBG structure is not applied can be used as the return current path of a main high-speed signal line. 
         [0049]    Based on this phenomenon,  FIGS. 7A to 7C  show an example of a structure in which an EBG structure of a different size is partially applied to the ground plane and the power plane, and DeCaps are placed at specific positions around a noise generating source in accordance with the embodiment of the present invention. 
         [0050]    Referring to  FIG. 7A , an electromagnetic wave suppression structure  702  and DeCaps  704  are partially placed together only around P 1  where a noise generating source is assumed to be at P 1  position. Referring to  FIG. 7B , an electromagnetic wave suppression structure  712  is partially placed only around P 2  where noise generating sources or noise sensitive devices are assumed to be at P 2  position. 
         [0051]    By this placement, as shown in  FIG. 7C , the electromagnetic wave suppression structure  702  and the DeCap  704  are partially placed together on the ground plane  700 , and the electromagnetic wave suppression structure  712  is partially placed on the power plane  710 . 
         [0052]    Meanwhile, the EBG unit cell structures, i.e., electromagnetic suppression structures  702  and  712 , placed on the ground plane  700  and the power plane  710  may have different electromagnetic wave suppression frequency bandwidths in order to widen the noise suppression frequency bandwidth, and the electromagnetic wave suppression structures  702  and  712  may be fully placed across the ground plane  700  and the power plane  710  without being limited to the corresponding port where the noise generating sources and/or the noise sensitive devices are present. 
         [0053]      FIGS. 8A and 8B  are views showing DeCaps placed only in the vicinity of a noise generating source and a triangular electromagnetic wave suppression structure in accordance with the embodiment of the present invention. 
         [0054]    Referring to  FIG. 8A , DeCaps  802  are placed at specific positions around a noise generating source and a triangular electromagnetic wave suppression structure  800  is fully applied across the power plane or the ground plane. Referring to  FIG. 8B , DeCaps  812  are disposed at specific positions around a noise generating source and a triangle electromagnetic wave suppression structure  810  is partially applied to the power plane or the ground plane. 
         [0055]    In this manner, the electromagnetic wave suppression structure can be implemented in the shape of various polygons, such as a rectangle, a square, a triangle, a lozenge depending on an implementation method of the EBG unit cells. 
         [0056]      FIGS. 9A to 9F  are views showing various methods of placing DeCaps in a multilayer board structure where an EBG structure is partially placed in accordance with the embodiment of the present invention. 
         [0057]    It is seen throughout  FIGS. 9A to 9F  that an electromagnetic wave suppression structure and DeCaps are partially placed together only around P 1  where the noise generating sources and/or the noise sensitive devices are present. 
         [0058]    Referring to  FIG. 9A , on a ground plane  900 , an electromagnetic wave suppression structure  902  and a DeCap  904  are partially placed around P 1 , and the DeCap  904  may be placed at a certain point of the circumference having a radius equal to a preset distance from the port P 1 . 
         [0059]    In  FIG. 9B , on a ground plane  910 , an electromagnetic wave suppression structure  912  and eight DeCaps  914  are partially placed around P 1 .  FIG. 9C  shows that on a ground plane  920 , an electromagnetic wave suppression structure  922  and two DeCaps  924  are partially placed around P 1 , and herein, the two DeCaps  924  are placed above and below the port P 1 . In  FIG. 9D , on a ground plane  930 , an electromagnetic wave suppression structure  932  and two DeCaps  934  are partially placed around P 1 , and herein, the two DeCaps  934  are placed on the left and right of the port P 1 . 
         [0060]    Referring to  FIG. 9E , on a ground plane  940 , an electromagnetic wave suppression structure  942  and four DeCaps  944  are partially placed around P 1 , and herein, the four DeCaps  944  are placed in a square shape with the port P 1  as the center. Finally referring to  FIG. 9F , on a ground plane  950 , an electromagnetic wave suppression structure  952  and four DeCaps  954  are partially placed around P 1 , and herein, the four DeCaps  954  are placed above and below and on the left and right of the port P 1 . 
         [0061]    Meanwhile, the DeCaps shown in  FIGS. 9A to 9F , by adjusting capacitance magnitudes and positions, can control the unwanted electromagnetic waves and noise suppression frequency band and noise suppression level. 
         [0062]    Therefore, the noise suppression characteristics can be optimized by selecting an optimum position in consideration of a frequency band to be suppressed, a noise suppression level, placement of parts on the substrate, and the like. Also, in order to adjust the unwanted electromagnetic waves and noise suppression frequency band and noise suppression level, an embedded DeCap having high dielectric constant and placed between the power plane and the ground plane can be used to increase the capacitance of the DeCap and reduce the parasitic inductance thereof. 
         [0063]    As described above, the multilayer board for suppressing unwanted electromagnetic waves and noise in accordance with the embodiment of the present invention has some effects as follows. 
         [0064]    First, unwanted wideband electromagnetic waves and noise generated in the multilayer board structure can be suppressed by DeCaps in a low frequency band and by a partially placed electromagnetic wave suppression structure in a frequency band more than several hundreds of MHz. As the electromagnetic wave suppression structure is partially placed in a specific area, a ground plane or power plane having no electromagnetic wave suppression structure applied thereto can be used as a return current path for high-speed signal lines while maintaining the noise suppression characteristics of the electromagnetic wave suppression structure, thereby improving signal characteristics of an entire system. 
         [0065]    Moreover, noise generation can be reduced by reducing self-impedance by applying both DeCaps and an EBG structure to a noise generating source. 
         [0066]    Further, it is possible to expand the suppression frequency bandwidth or properly adjust the noise suppression level by varying the shape or size of a partially placed electromagnetic wave suppression structure and the position and size of DeCaps. Therefore, an optimum noise suppression environment for performance improvement can be provided depending on the characteristics of a product to be applied. 
         [0067]    While the invention has been shown and described with respect to the embodiments, it will be understood by those skilled in the art that various changes and modification may be made without departing from the scope of the invention as defined in the following claims.