Method and means for decoupling a printed circuit board

[0002] 1. Field of the Invention

[0003] The present invention relates in general to printed circuit boards and, in particular, to an improved method and means for decoupling parallel planes included within a printed circuit board. Still more particularly, the present invention relates to a method and means for decoupling parallel planes included within a printed circuit board utilizing a first plurality of decoupling elements in a first region of the planes and a second plurality of decoupling elements in a second region of the planes.

[0004] 2. Description of the Related Art

[0005] The inclusion of a capacitive decoupling network between the power and ground planes of a printed circuit board (PCB) is well known and desirable for a number of reasons. Specifically, decoupling capacitors may be provided in the form of discrete decoupling capacitors connected between the power and ground planes of the PCB, the discrete decoupling capacitors being located adjacent to loads, which typically comprise integrated circuits (ICs) mounted on the PCB. Decoupling capacitance is also provided in the form of intrinsic capacitance by the parallel location of the power and ground planes with a PCB. This decoupling capacitance serves to provide fast rise time current to the integrated circuit (IC) loads and to minimize differential voltages that may develop between the power and ground planes of the PCB as a result of IC switching events by isolating the switching currents as local events. This in turn reduces the potential of the PCB to radiate and conduct electromagnetic interference (EMI).

[0006] FIG. 1 shows an equivalent circuit 10 for an integrated circuit 12 and a decoupling capacitor (C_d) 14 mounted on a PCB. The intrinsic, or interplanar, capacitance is represented by the capacitor (C₀) 16. The power plane is represented by the line 18, and the ground plane by line 20.

[0007] The power plane of a PCB should ideally exhibit zero impedance. It is desirable to minimize the impedance of power planes as much as possible. FIG. 2 is a graph showing power plane impedance (Z) versus frequency (MHz) for an example power and ground plane pair within a PCB having a single type of discrete decoupling capacitors mounted thereon. The graph plots the impedance of the power plane, with the PCB configured as a “bare board” (i.e. a PCB without any discrete capacitors mounted thereon), with broken line 22, and the impedance of the power plane, with the PCB having the single type of discrete decoupling capacitors mounted thereon, with solid line 24. As is apparent from the graph, the impedance increases dramatically at operating frequencies above a series resonant frequency (f₁) due, inter alia, to the presence of the discrete decoupling capacitors, and the impedance theoretically becomes infinite at a parallel resonant frequency (f₂). The impedance of a PCB employing a single type of discrete decoupling capacitors comprises the cumulative impedance of the power and ground planes, vias connecting these planes, traces between capacitors and ICs, and the capacitor mounting pads. The significant increases in the power plane impedance at high frequencies results in the performance of the PCB deteriorating substantially at these frequencies, at which the PCB may not function properly, or may exceed regulatory emission levels.

[0008] The series resonant frequency (f₁) is determined mainly by the number and location of the single type of discrete decoupling capacitors. By optimizing the number and location of the single type of discrete decoupling capacitors, as well as other PCB characteristics, it is possible to shift the series resonant frequency (f₁) to higher frequencies as increased operating frequencies are encountered. However, PCB operating frequencies are being achieved at which it has become increasingly difficult, inefficient, and expensive to match the series resonant frequency (f₁) to the PCB operating frequency. Computers are now operating at clock rates much higher than was possible in the past. This results in the creation of high frequency harmonic energy in the hundreds of megahertz, and even into the gigahertz region.

[0009] A “Q-factor” may be determined for a resonant structure, such as a structure included within a printed circuit board. The “Q-factor” of a resonant structure is based on the amount of loss in the structure. The higher the amount of loss, the lower the “Q-factor” becomes. In the case of a simple parallel power and ground plane structure there is little loss. Therefore, the “Q-factor” is high. This would normally result in higher EMC emissions.

[0010] Therefore a need exists for an improved method and means for decoupling parallel planes included within a printed circuit board both at low-level as well as high-level frequencies.

[0011] An improved method and means for decoupling a printed circuit board are disclosed. A power plane is included having a peripheral edge. The power plane includes a first region and a second region which is separate from and contiguous to the first region. The first region is located from the peripheral edge to a middle portion of the power plane. The first region includes a peripheral portion of the power plane. The second region includes only the middle portion of the power plane. A ground plane is coupled in parallel to the power plane. The ground plane has a peripheral edge. The ground plane includes a first region, and a second region which is separate from and contiguous to the first region. The first region includes the peripheral edge and includes a peripheral portion of the ground plane. The second region includes a middle portion of the ground plane. A first plurality of decoupling elements are connected to the first region of the power plane and to the first region of the ground plane. A second plurality of decoupling elements are connected to the second region of the power plane and to the second region of the ground plane. The first and second plurality of decoupling elements are utilized to decouple the power plane and ground plane pair.

[0012] The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.

[0013] The novel features are set forth in the appended claims. The present invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment when read in conjunction with the accompanying drawings, wherein:

[0014] FIG. 1 depicts an equivalent circuit representing an integrated circuit (IC) and a single type of decoupling capacitor mounted on a printed circuit board (PCB) in accordance with the prior art;

[0015] FIG. 2 illustrates a graph plotting impedance of a power plane versus frequency in accordance with the prior art;

[0016] FIG. 3 is a pictorial representation of a power plane and a ground plane separated by a dielectric layer in accordance with the method and system of the present invention;

[0017] FIG. 4 is a schematic diagram of a power plane, a ground plane, a first plurality of decoupling elements connected to a first region of the planes, and a second plurality of decoupling elements connected to a second region of the planes in accordance with the method and system of the present invention; and

[0018] FIG. 5 illustrates a perspective view of a power plane and a ground plane including removed portions of conductors on these planes in accordance with the method and system of the present invention.

[0019] A preferred embodiment of the present invention and its advantages are better understood by referring to FIGS. 1-5 of the drawings, like numerals being used for like and corresponding parts of the accompanying drawings.

[0020] The present invention is an improved printed circuit board for decoupling a pair of planes included within the printed circuit board (PCB). A generally solid power plane and a generally solid ground plane are included in parallel within the PCB. The solid power and ground planes are conductors in themselves and are typically formed from a conducting material such as copper. The power plane includes two regions. The first region is located along the peripheral edge of the power plane and extends from the edge toward a middle of the plane. The second region adjoins the first region and includes only the middle area of the plane.

[0021] Similarly, the ground plane includes two regions. The first region is located along the peripheral edge of the ground plane and extends from the edge toward a middle of the plane. The second region adjoins the first region and includes only the middle area of the ground plane.

[0022] A first plurality of decoupling elements are connected to the first region of the power plane and the first region of the ground plane to decouple the planes. A second plurality of decoupling elements are connected to the second region of the power plane and the second region of the ground plane to decouple the planes.

[0023] The first plurality of decoupling elements includes two types of decoupling capacitors, while the second plurality of decoupling elements includes only one type of decoupling capacitors. In a preferred embodiment, the first plurality of decoupling elements includes both low equivalent series resistance (ESR) capacitors as well as lossy capacitors. The second plurality of decoupling elements includes only lossy capacitors.

[0024] In addition, portions of the conductors of either the power plane, the ground plane, or both planes are moved. Therefore, the planes will not conduct in these removed portions. Multiple, rectangular portions of a conductor of a plane are removed. Each portion is parallel to the other portions removed from the plane. Further, the portions removed from one plane are preferably perpendicular to the portions removed from the other plane.

[0025] The present invention, therefore, uses a combination of low ESR capacitors and lossy decoupling capacitors coupled to different regions of the parallel planes to significantly reduce or eliminate high frequency resonances without losing the low frequency performance of the ESR decoupling capacitors. This reduction in high frequency resonances results in a significant reduction in EMC emissions and signal integrity, i.e. signal functional, problems due to these resonances.

[0026] FIG. 3 is a pictorial representation of a generally solid plane 32 utilized to provide power and a generally solid plane as a ground plane 34 separated by a dielectric layer 36 in accordance with the method and system of the present invention. Power plane 32 and ground plane 34 are located in parallel within a printed circuit board. Dielectric layer 36 separates power plane 32 and ground plane 34 and provides electrical isolation between power plane 32 and ground plane 34. Planes 32, 34 are typically formed from copper. Dielectric layer 36 is typically formed from an insulating material such as epoxy-fiberglass.

[0027] Power plane 32, ground plane 34, and dielectric layer 36 are included within a printed circuit board to provide power to the devices within the printed circuit board. A multi-layer PCB typically includes multiple power and ground planes. The present invention will be described with reference to a single power plane and ground plane pair. However, those skilled in the art will recognize that the present invention may also be utilized in PCBs having multiple power/ground plane pairs.

[0028] Power plane 32 and ground plane 34 are each divided into multiple regions. In the preferred embodiment, each plane is divided into two separate and distinct regions. A first region includes an area from the peripheral edge of the plane in toward the center of the plane. In the preferred embodiment, the first region is a rectangular region extending inward from the peripheral edge of the plane. The second region includes the remainder of the plane which includes a middle area of the plane.

[0029] Power plane 32 includes a first region 38 and a second region 40. First region 38 extends from a peripheral edge 42 of power plane 32 toward a line 44 which divides first region 38 from second region 40. Second region 40 extends from line 44 toward a middle 46 of plane 32.

[0030] FIG. 4 is a schematic diagram of a power plane, a ground plane, and first and second plurality of decoupling elements in accordance with the method and system of the present invention. Power plane 32 includes a first region 38 and a second region 40. Similarly, ground plane 34 includes a first region 44 and a second region 46. First regions 38 and 44 and second regions 40 and 46 are similarly shaped and located on their respective planes.

[0031] A first plurality of decoupling elements 48 are connected to first regions 38 and 44. A second plurality of decoupling elements 50 are connected to second regions 40 and 46. In this manner, first decoupling elements 48 are connected to power plane 32 and ground plane 34 along a peripheral area of planes 32 and 34. Preferably, first decoupling elements 48 are coupled to planes 32 and 34 in a generally rectangularly-shaped frame. Second decoupling elements 50 are connected to power plane 32 and ground plane 34 in a solid, middle, rectangular area of planes 32 and 34.

[0032] First decoupling elements 48 include two types of decoupling elements, low equivalent series resistance (ESR) capacitors and lossy capacitors. The ESR capacitors, such as capacitor 52, provide good decoupling performance at low frequencies, typically below 400 MHz. The lossy capacitors, such as the series resistance and capacitor element 54, significantly lower the “Q” factor of the resonant structure. In a preferred embodiment, approximately 40-60% of the total decoupling elements 48 should be lossy capacitors. The remaining decoupling elements 48 should be ESR capacitors.

[0033] Second decoupling elements 50 include only one type of decoupling element, i.e. lossy capacitors. Lossy capacitors, such as element 54, are utilized for all of decoupling elements 50.

[0034] FIG. 5 illustrates a perspective view of power plane 32 and ground plane 34 including removed portions of conductors on these planes in accordance with the method and system of the present invention. To further alter the resonant structure of the power/ground plane pair, portions of the conductors are removed from each plane. A plurality of portions 60 of the conductor of power plane 32 are removed, and a plurality of portions 62 of the conductor of ground plane 34 are removed. The plane will not conduct in these removed portions.

[0035] In accordance with an important feature of the present invention, the portions are removed from only the second regions of each board. Therefore, portions 60 are removed from region 40, and portions 62 are removed from region 46. In this manner, portions 60, 62 do not extend to an edge of either plane.

[0036] Preferably, each portion is a generally rectangularly-shaped portion positioned parallel to each other removed portion on the plane. Therefore, within a single plane, the conductor is removed in parallel, rectangular portions from the plane.

[0037] In accordance with another important feature of the present invention, the portions removed from one plane are perpendicular to the portions removed from the other plane. Therefore, portions 60 are perpendicular to portions 62.

[0038] The resonance is due in part to the size of the planes. By removing portions of the conductor of a plane, the resonance frequency of the plane is adjusted to a higher frequence, well beyond the frequencies of concern. The current must flow in a particular manner in order for the standing wave to form. By removing portions of the conductor, the standing wave cannot form. None of the conductor near the edge of the plane is removed, however. Therefore, the DC voltage remains constant on the planes.

[0039] The present invention uses a combination of three different approaches to reduce or eliminate high frequency resonances. The low ESR capacitors provide good decoupling performance at lower frequencies below 400 MHz. The lossy capacitors, including a capacitor and resistor in series, significantly lower the “Q factor” of the resonant structure, thereby greatly reducing the peak energy contained within the resonances. Removing portions of the conductors in one or more of the planes changes the size of the resonant structure, effectively moving the resonant frequencies to frequencies which are much higher, typically 10-100 GHz. Therefore, the potential resonances in the frequencies from 400 to 1-5 GHz are either eliminated or greatly reduced.

[0040] While a preferred embodiment has been particularly shown and described, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention.